doc: afs_compile_et -h option

Document the afs_compile_et -h option.

Reviewed-on: http://gerrit.openafs.org/10722
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 501d6d288fcaa195c3cda0be0e3423c7959c9958)

Change-Id: I5085f35d1b75a6dbae2768c2ba1ea61bb9c4201c
Reviewed-on: http://gerrit.openafs.org/11226
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>