documentation for XLALFindFiles()

@karl-wette I've noticed that XLALFindFiles() has no doxygenised documentation. As a first step, I could simply reformat the current source code documentation block:

  * glob() has been reported to fail under condor, so we use our own
  * function to get a filelist from a directory, using a glob-like pattern.
  * also can read a list of file names from a "list file".
  *
  * looks pretty ugly with all the #ifdefs for the Microsoft C compiler
  *
  * NOTE: the list of filenames is returned SORTED ALPHABETICALLY !
  *

(Probably dropping the note about #ifdefs, there are much uglier codes in LALSuite. ;))

However, skimming through the code, there also seem to be a few special modes such as the "list file". Do you understand this function well enough to add more details on what it can do?