webapi.txt: overhaul documentation. API changes are as follows:

webapi.txt: overhaul documentation. API changes are as follows:

 * download/upload localdir=/localfile= has been removed. This sort of ambient
   authority was unsafe to expose over the web (CSRF), and at some point
   soon we'll have 'cp -r' in the CLI to replace it.
 * GET save=filename -> GET filename=filename&save=true
 * GET t=download removed
 * side-effect causing operations now use POST where appropriate, not PUT
 * to create multiple directories, either use
   * POST /uri/DIRCAP/parent?t=mkdir&name=child  (more form/browser oriented)
   * POST /uri/DIRCAP/parent/child?t=mkdir (more machine oriented)
   The t=mkdir-p form is still accepted, but not preferred (since it leaks
   the child name queryarg into the logs)
 * use PUT /uri/MUTABLEFILECAP or PUT /uri/DIRCAP/child (on a mutable file) to
   replace its contents, or POST /same?t=upload from forms
 * response bodies and codes are better specified than before