website: Document division behavior change in v0.12 upgrade guide

The division operator now always performs floating point division, whereas
before it would choose between float and int division based on the types
of its arguments.

We have a specific error message for when a fractional number is used as
an index in HCL, but this additional upgrade guidance provides a specific
solution to the problem: the floor function.

Sadly we don't have enough context in the current design of the upgrade
tool to make this fix automatic. With some refactoring it may be possible
to apply the fix automatically within list brackets, but since that is
a relatively complex change we'll first try this manual solution prompted
by an error message, because in practice so far we've seen this reported
only in the context of list indexing and our error check will catch that
and make the user aware of the need for a fix there.

website/upgrade-guides/0-12.html.markdown

@@ -237,7 +237,46 @@ into a list. The upgrade tool does not remove or attempt to consolidate
 any existing duplicate arguments, but other commands like `terraform validate`
 will detect and report these after upgrading.
 
+## Integer vs. Float Number Types
+
+From Terraform v0.12, the Terraform language no longer distinguishes between
+integer and float types, instead just having a single "number" type that can
+represent high-precision floating point numbers. This new type can represent
+any value that could be represented before, plus many new values due to the
+expanded precision.
+
+In most cases this change should not cause any significant behavior change, but
+please note that in particular the behavior of the division operator is now
+different: it _always_ performs floating point division, whereas before it
+would sometimes perform integer division by attempting to infer intent from
+the argument types.
+
+If you are relying on integer division behavior in your configuration, please
+use the `floor` function to obtain the previous result. A common place this
+would arise is in index operations, where the index is computed by division:
+
+```hcl
+  example = var.items[floor(count.index / var.any_number)]
+```
+
+Using a fractional number to index a list will produce an error telling you
+that this is not allowed, serving as a prompt to add `floor`:
+
+```
+Error: Invalid index
+
+The given key does not identify an element in this collection value: indexing a
+sequence requires a whole number, but the given index (0.5) has a fractional
+part.
+```
+
+Unfortunately the automatic upgrade tool cannot apply a fix for this case
+because it does not have enough information to know if floating point or integer
+division was intended by the configuration author, so this change must be made
+manually where needed.
+
 ## Terraform Configuration upgrades requiring human intervention 
+
 There are some known situations that will be detected, but not upgrade, by the
 upgrade tool. Some examples of these situatations include: