`portfwd-loopback-choice' was not consistently documented.

`portfwd-loopback-choice' was not consistently documented.
 - update usage info in tools
   - ack, plink is over 24 lines now
 - update man pages for Unix version
 - Doc changes:
   - move long description from (GUI) "config" to "using"
     - sorry if complete specification isn't what this section is meant for,
       but if you only read "using" it was hard to find.
   - ensure enough references to this made in other sections (GUI,
     command-line)
   - update instance of plink usage info

git-svn-id: svn://svn.tartarus.org/sgt/putty@3740 cda61777-01e9-0310-a592-d414129be87e