Add a language pack upgrader class.

At the conclusion of any upgrade, after the transients are refreshed from the API, pending translations are downloaded and installed to wp-content/languages.

props dd32.
see #18200.

Built from https://develop.svn.wordpress.org/trunk@25566


git-svn-id: http://core.svn.wordpress.org/trunk@25484 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Inline documentation improvements for wp-includes/comment-template.php.

Changes include:
- Existing phpdoc block improvements
- Inclusion of defaults for optional parameters
- Hash-notated argument arrays
- Removal of unnecessary or redundant tagging.

Also standardization of int|WP_Post function parameters.

These changes clear the way toward less redundancy for hook docs in a future ticket.

Still left: Fully documented arguments in `comment_form()` and a pass through Walker_Comment.

See #20495. See #25388.

Built from https://develop.svn.wordpress.org/trunk@25567


git-svn-id: http://core.svn.wordpress.org/trunk@25485 1a063a9b-81f0-0310-95a4-ce76da25c4cd