doc: Converting GFID to Path.

This is an attempt to document various possible ways to externally
get file/dir path from its GFID.

Change-Id: Ic31725a9706ddae8972aea9bb58f3b70978dbd06
Signed-off-by: Prashanth Pai <ppai@redhat.com>
Reviewed-on: http://review.gluster.org/8552
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>