StringUtils: Fixup Doxygen warnings

StringUtils: Fixup Doxygen warnings

* Code blocks must be indented by 4 spaces (so 5 in total from the star).

* Code blocks are only supported in descriptions, and for some reason the first
  line is separate from the description, so there must be an additional line of
  text between the summary and the code block since it can't be the first thing
  in a description.

* Fix regex confusion with comment syntax due to "/*\/".

To verify these findings, use:

    $ php maintenance/mwdocgen.php --file includes/libs/StringUtils.php

And check the output warnings and generated html pages.

Change-Id: I259b87c0bf3b48a95df0108eb6ee799c25b2853f