website: "coalesce" function unifies its argument types

In order to be able to predict a result type even if arguments are not yet
known, coalesce requires all of its arguments to be of the same type. Our
usual automatic conversion rules mean that in some cases the result is
a silent type conversion rather than an explicit error, so we'll at least
document that so that folks who encounter it can understand what is
causing the likely-surprising behavior.

If we were building this function over again today I expect we'd make it
always return an error under type mismatch, but to do so now would be a
breaking change and the potential cost of that seems too high for
something that doesn't seem to arise incredibly often in practice.

website/docs/configuration/functions/coalesce.html.md

@@ -16,6 +16,11 @@ earlier, see
 `coalesce` takes any number of arguments and returns the first one
 that isn't null or an empty string.
 
+All of the arguments must be of the same type. Terraform will try to
+convert mismatched arguments to the most general of the types that all
+arguments can convert to, or return an error if the types are incompatible.
+The result type is the same as the type of all of the arguments.
+
 ## Examples
 
 ```
@@ -35,6 +40,22 @@ symbol to expand the list as arguments:
 b
 ```
 
+Terraform attempts to select a result type that all of the arguments can
+convert to, so mixing argument types may produce surprising results due to
+Terraform's automatic type conversion rules:
+
+```
+> coalesce(1, "hello")
+"1"
+> coalesce(true, "hello")
+"true"
+> coalesce({}, "hello")
+
+Error: Error in function call
+
+Call to function "coalesce" failed: all arguments must have the same type.
+```
+
 ## Related Functions
 
 * [`coalescelist`](./coalescelist.html) performs a similar operation with