Improved documentation in l10n.php

[johnbillion]

Previously: #34127

[johnbillion]

37585.patch​ does the following:

• Changes $translations to $translation in the gettext and gettext_with_context filter parameters so they match those of ngettext and ngettext_with_context. The singular form is the correct terminology.
• Adds example code that's actually useful to _nx(), _nx_noop(), and _n_noop().
• Simplifies example code in _n() and translate_nooped_plural().

[wonderboymusic]

In 38360:

L10N: improve accuracy of docs.

Props johnbillion.
Fixes #37585.