Update @warning doc to clarify +/- (#873)

Co-authored-by: Jimmy Timmons <jtimmons@autobooks.co>

Author: jimmytimmons

misc_docs/syntax/decorator_expression_warning.mdx

@@ -6,7 +6,7 @@ summary: "This is the `@warning` decorator."
 category: "decorators"
 ---
 
-The `@warning` decorator is used to modify the enabled compiler warnings for types, values and submodules. See [here](/docs/manual/latest/warning-numbers) for all available warning numbers.
+The `@warning` decorator is used to modify the enabled compiler warnings for types, values and submodules. See [here](/docs/manual/latest/warning-numbers) for all available warning numbers. Prefix the warning number with `+` or `-` to enable or disable respectively.
 
 ### Examples