configs: A better error message for custom variable validation

Our previous message conflated the requirement for a full sentence with
the suggestion to write in a style similar to Terraform's built-in error
messages, which created a sense that the system would actively reject an
error message written in another language.

There's no intent here to block writing error messages in other languages,
but there is a practical consideration that Terraform's UI output is
currently not localized and so consistency with Terraform's other output,
if that's important to a module author, will typically mean writing the
error message in English.

configs/named_values.go

@@ -382,7 +382,7 @@ func decodeVariableValidationBlock(varName string, block *hcl.Block, override bo
 				diags = diags.Append(&hcl.Diagnostic{
 					Severity: hcl.DiagError,
 					Summary:  errSummary,
-					Detail:   "Validation error message must be at least one full English sentence starting with an uppercase letter and ending with a period or question mark.",
+					Detail:   "The validation error message must be at least one full sentence starting with an uppercase letter and ending with a period or question mark.\n\nYour given message will be included as part of a larger Terraform error message, written as English prose. For broadly-shared modules we suggest using a similar writing style so that the overall result will be consistent.",
 					Subject:  attr.Expr.Range().Ptr(),
 				})
 			}