doc: afs_compile_et -h option

Document the afs_compile_et -h option.

Change-Id: I4972bcc1948e8dd7ae73dfcabfbaf822cfbfe64b
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>