Documentation: remove source code comments covered elsewhere (wikitext, 6574117)

Remove redundant RDoc comments from the main wikitext module source file as these are now covered in the separate RDoc file in the doc subdirectory.

At the same time update the definition of "treat_slashes_as_special" to reflect the new, stricter definition of what classifies as a special link.

Signed-off-by: Greg Hurrell <greg@hurrell.net>