Add minimal rlm_krb5 documentation file

Add doc/modules/rlm_krb5 - a minimal rlm_krb5 module documentation file,
based on the wiki page. Update raddb/mods-available/krb5 comments to
point to the actual and proper location.
Merge pull request #736 from spbnick/add_rlm_krb5_doc

Add minimal rlm_krb5 documentation file