FileNameSniff: Document the additional checks in the ruleset
FileNameSniff: Document the additional checks in the ruleset

Also:
* Remove the now superfluous `Generic.Files.LowercasedFilename` sniff
* Excluding this sniff from the ruleset for WPCS itself

WPTRT/WordPress-Coding-Standards,WPTRT/WordPress-Coding-Standards,WPTRT/WordPress-Coding-Standards,WPTRT/WordPress-Coding-Standards,WPTRT/WordPress-Coding-Standards