Update docblocks for l10n.php

[johnbillion]

34127.diff​ addresses several issues with the docblocks for our l10n functions.

• Corrects phrasing used in the descriptions for and filters in _n() and _nx(), particularly with regard to the use of 'singular' instead of 'single' or '1', and 'plural' instead of 'not 1'.
• Adds example usage to _n(), _nx(), _nx_noop(), and translate_nooped_plural().
• Adds hash notation @return doc for _n_noop() and _nx_noop().
• Adds missing default values for the $domain parameter.

[DrewAPicture]

In 34781:

Docs: Improve a variety of inline docs in wp-includes/l10n.php.

• Corrects phrasing used in the descriptions for and filters in _n() and _nx(), particularly with regard to the use of 'singular' instead of 'single' or '1', and 'plural' instead of 'not 1'.
• Adds example usage to _n(), _nx(), _nx_noop(), and translate_nooped_plural().
• Adds hash notation @return doc for _n_noop() and _nx_noop().
• Adds missing default values for the optional $domain parameter in several places.

Props johnbillion.
Fixes #34127.