Fix RefTeX to show table of contents for dtx files (tiny change)

Fix RefTeX to show table of contents for dtx files (tiny change)

* lisp/textmodes/reftex.el (reftex-compile-variables): Change the
  section regexp so that it accepts lines starting with the comment
  character. (tiny change)
* lisp/textmodes/reftex-parse.el (reftex-parse-from-file): Filter
  gathered toc entries, accepting a commented entry if and only if the
  source file is a ".dtx" file. (tiny change)