diff --git a/website/docs/configuration/expressions.html.md b/website/docs/configuration/expressions.html.md
index 040e2f732..1d0e1bdc4 100644
--- a/website/docs/configuration/expressions.html.md
+++ b/website/docs/configuration/expressions.html.md
@@ -578,6 +578,17 @@ together results that have a common key:
 {for s in var.list : substr(s, 0, 1) => s... if s != ""}
 ```
 
+For expressions are particularly useful when combined with other language
+features to combine collections together in various ways. For example,
+the following two patterns are commonly used when constructing map values
+to use with [resource `for_each`](./resources.html#for_each-multiple-resource-instances-defined-by-a-map-or-set-of-strings):
+
+* Transform a multi-level nested structure into a flat list by
+  [using nested `for` expressions with the `flatten` function](./functions/flatten.html#flattening-nested-structures-for-for_each).
+* Produce an exhaustive list of combinations of elements from two or more
+  collections by
+  [using the `setproduct` function inside a `for` expression](./functions/setproduct.html#finding-combinations-for-for_each).
+
 ## Splat Expressions
 
 A _splat expression_ provides a more concise way to express a common
diff --git a/website/docs/configuration/resources.html.md b/website/docs/configuration/resources.html.md
index c2116a196..981772d10 100644
--- a/website/docs/configuration/resources.html.md
+++ b/website/docs/configuration/resources.html.md
@@ -376,6 +376,27 @@ This object has two attributes:
 - `each.value` — The map value corresponding to this instance. (If a set was
   provided, this is the same as `each.key`.)
 
+#### Using Expressions in `for_each`
+
+The `for_each` meta-argument accepts map or set [expressions](./expressions.html).
+However, unlike most resource arguments, the `for_each` value must be known
+_before_ Terraform performs any remote resource actions. This means `for_each`
+can't refer to any resource attributes that aren't known until after a
+configuration is applied (such as a unique ID generated by the remote API when
+an object is created).
+
+The `for_each` value must be a map or set with one element per desired
+resource instance. If you need to declare resource instances based on a nested
+data structure or combinations of elements from multiple data structures you
+can use Terraform expressions and functions to derive a suitable value.
+For example:
+
+* Transform a multi-level nested structure into a flat list by
+  [using nested `for` expressions with the `flatten` function](./functions/flatten.html#flattening-nested-structures-for-for_each).
+* Produce an exhaustive list of combinations of elements from two or more
+  collections by
+  [using the `setproduct` function inside a `for` expression](./functions/setproduct.html#finding-combinations-for-for_each).
+
 #### Referring to Instances
 
 When `for_each` is set, Terraform distinguishes between the resource block itself
@@ -396,16 +417,19 @@ as a whole.
 #### Using Sets
 
 The Terraform language doesn't have a literal syntax for
-[sets](./types.html#collection-types), but you can use the `toset` function to
-convert a list of strings to a set:
+[set values](./types.html#collection-types), but you can use the `toset`
+function to explicitly convert a list of strings to a set:
 
 ```hcl
-variable "subnet_ids" {
-  type = list(string)
+locals {
+  subnet_ids = toset([
+    "subnet-abcdef",
+    "subnet-012345",
+  ])
 }
 
 resource "aws_instance" "server" {
-  for_each = toset(var.subnet_ids)
+  for_each = local.subnet_ids
 
   ami           = "ami-a1b2c3d4"
   instance_type = "t2.micro"
@@ -417,24 +441,26 @@ resource "aws_instance" "server" {
 }
 ```
 
-#### Using Expressions in `for_each`
+Conversion from list to set discards the ordering of the items in the list and
+removes any duplicate elements. `toset(["b", "a", "b"])` will produce a set
+containing only `"a"` and `"b"` in no particular order; the second `"b"` is
+discarded.
 
-The `for_each` meta-argument accepts map or set [expressions](./expressions.html).
-However, unlike most resource arguments, the `for_each` value must be known
-_before_ Terraform performs any remote resource actions. This means `for_each`
-can't refer to any resource attributes that aren't known until after a
-configuration is applied (such as a unique ID generated by the remote API when
-an object is created).
+If you are writing a module with an [input variable](./variables.html) that
+will be used as a set of strings for `for_each`, you can set its type to
+`set(string)` to avoid the need for an explicit type conversion:
 
-The `for_each` value must be a map or set with one element per desired
-resource instance. If you need to declare resource instances based on a nested
-data structure or combinations of elements from multiple data structures you
-can use Terraform expressions and functions to derive a suitable value.
-For some common examples of such situations, see the
-[`flatten`](/docs/configuration/functions/flatten.html)
-and
-[`setproduct`](/docs/configuration/functions/setproduct.html)
-functions.
+```
+variable "subnet_ids" {
+  type = set(string)
+}
+
+resource "aws_instance" "server" {
+  for_each = var.subnet_ids
+
+  # (and the other arguments as above)
+}
+```
 
 ### `provider`: Selecting a Non-default Provider Configuration