website: Dynamic blocks can for_each any collection type

We previously added a hint to both resource for_each and dynamic blocks
about using the "flatten" and "setproduct" situations to construct
suitable collections to repeat over.

However, we used the same text in both places which ended up stating that
dynamic blocks can only accept map or set values, which is a constraint
that applies to resource for_each (because we need to assign a unique
identifier to each instance) and not to dynamic blocks (which don't have
any uniqueness enforced by Terraform Core itself).

To remove that contradiction with the text above which talks about what
is valid here, I've just generalized this to say "collection", because
the primary point of this paragraph is the "one element per desired nested
block" part, not specifically what sort of collections are permitted in
this location. (Text further up describes the supported types.)

website/docs/language/expressions/dynamic-blocks.html.md

@@ -78,7 +78,7 @@ to generate meta-argument blocks such as `lifecycle` and `provisioner`
 blocks, since Terraform must process these before it is safe to evaluate
 expressions.
 
-The `for_each` value must be a map or set with one element per desired
+The `for_each` value must be a collection with one element per desired
 nested block. 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.