Extract: add a phpdoc to clarify the class responsibility

First step towards making it do only one thing and cleaning it up


git-svn-id: https://develop.svn.wordpress.org/trunk@25917 602fd350-edb4-49c9-b593-d223f7449a82
Extract: add default rules

Two purposes:

 * Helps people, reading the code to have a better idea of the structure of the array
 * Reciprocity with `$comment_prefix`, which has a default value

See #25691


git-svn-id: https://develop.svn.wordpress.org/trunk@25918 602fd350-edb4-49c9-b593-d223f7449a82
Extract: `_` and `_c` aren't among the common gettext function names

Let's just remove them from the default rules


git-svn-id: https://develop.svn.wordpress.org/trunk@25919 602fd350-edb4-49c9-b593-d223f7449a82