doc: afs_compile_et -lang short form

doc: afs_compile_et -lang short form

Document the -lang short form for -language.

Reviewed-on: http://gerrit.openafs.org/10720
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 355ca7b20b8bb9f06b030be966311d4afb40ed00)

Change-Id: I26c56ac064968d65f06de9f34c50bc71d2208560
Reviewed-on: http://gerrit.openafs.org/11223
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>