website: Update docs for check rule error messages

Author: alisdair

website/docs/language/expressions/preconditions-postconditions.mdx

@@ -335,11 +335,15 @@ Error: Resource postcondition failed
 The selected AMI must be tagged with the Component value "nomad-server".
 ```
 
-The `error_message` argument must always be a literal string, and should
-typically be written as a full sentence in a style similar to Terraform's own
-error messages. Terraform will show the given message alongside the name
-of the resource that detected the problem and any outside values used as part
-of the condition expression.
+The `error_message` argument can be any expression which evaluates to a string.
+This includes literal strings, heredocs, and template expressions. Multi-line
+error messages are supported, and lines with leading whitespace will not be
+word wrapped.
+
+Error message should typically be written as one or more full sentences in a
+style similar to Terraform's own error messages. Terraform will show the given
+message alongside the name of the resource that detected the problem and any
+outside values used as part of the condition expression.
 
 ## Preconditions or Postconditions?
 

website/docs/language/values/variables.mdx

@@ -198,10 +198,13 @@ variable "image_id" {
 ```
 
 If `condition` evaluates to `false`, Terraform will produce an error message
-that includes the sentences given in `error_message`. The error message string
+that includes the result of the `error_message` expression. The error message
 should be at least one full sentence explaining the constraint that failed,
 using a sentence structure similar to the above examples.
 
+Error messages can be literal strings, heredocs, or template expressions. The
+only valid reference in an error message is the variable under validation.
+
 Multiple `validation` blocks can be declared in which case error messages
 will be returned for _all_ failed conditions.