Make it clearer why these methods have a bang

An `!` means caution, but not much more. These comments help highlight what to be cautious about.

Co-authored-by: mattzollinhofer <matt.zollinhofer@gmail.com>

lib/pundit/context.rb

@@ -55,7 +55,7 @@ def policy_scope(scope)
       policy_scope.resolve
     end
 
-    # Retrieves the policy scope for the given record.
+    # Retrieves the policy scope for the given record. Raises if not found.
     #
     # @see https://github.com/varvet/pundit#scopes
     # @param user [Object] the user that initiated the action
@@ -90,7 +90,7 @@ def policy(record)
       raise InvalidConstructorError, "Invalid #<#{policy}> constructor is called"
     end
 
-    # Retrieves the policy for the given record.
+    # Retrieves the policy for the given record. Raises if not found.
     #
     # @see https://github.com/varvet/pundit#policies
     # @param user [Object] the user that initiated the action