Modify sniff to allow $ (this) in @return to not have a comment

Problem/Motivation

#2270551: Modify sniff to allow $ (this) in @return allowed $this as a type for @return,
but also we need to allow a comment under that to be optional

From the standard: https://www.drupal.org/node/1354#types
"You may omit the description if using @return $this"

Proposed resolution

Change the rules to allow @return $this to not have a comment

Remaining tasks

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User interface changes

None

API changes

None