I18N: Use underscores instead of dashes and interpolation instead of...

I18N: Use underscores instead of dashes and interpolation instead of concatenation in domain-specific gettext hooks, per the coding standards:

* `gettext_{$domain}`
* `gettext_with_context_{$domain}`
* `ngettext_{$domain}`
* `ngettext_with_context_{$domain}`

Additionally:
* Pass `$domain` parameter to the new hooks, for consistency with their pre-existing counterparts.
* Update documentation to explain the dynamic portion of the filter, for consistency with other dynamic hooks in core.

Follow-up to [48131].

Props swissspidy, knutsp, TimothyBlynJacobs, SergeyBiryukov.
Fixes #49518.
Built from https://develop.svn.wordpress.org/trunk@48136


git-svn-id: https://core.svn.wordpress.org/trunk@47905 1a063a9b-81f0-0310-95a4-ce76da25c4cd