Twenty Nineteen & Twenty Twenty One: Add PHPDoc Block for the twentynineteen_content_width & twenty_twenty_one_content_width filter

[pmbaldha]

Trac ticket: https://core.trac.wordpress.org/ticket/63664

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[sabernhardt]

I think these three single-line comments can be removed from both themes. The WPCS issue is not open anymore, and WordPress/WordPress-Coding-Standards#1773 indicates that the test makes an exception for $content_width.