Update @since tags to match documentation standards

[thebrandonallen]

I wrote a bash script to parse out the revision number, used by the previous standard, then check for it's earliest existence in a SVN tag, and run a find and replace. The result should be pretty accurate. The patch also adds breathing room around the @since tags, where appropriate. I've also updated a few @deprecated tags.

[netweb]

In 5951:

Standardize @since PHPDoc tags style and format

• e.g. @since 2.6.0 bbPress (r5951) YOLO
• See ​http://phpdoc.org/docs/latest/references/phpdoc/tags/since.html

This changeset updates the style and formatting of all of our @since instances for compatibility with phpDocumentor 2 and WP-Parser as we move toward implementing the bbPress developer code reference.

Props thebrandonallen
Fixes #2879