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