Document --enable-linux-d_splice_alias-extra-iput

Document --enable-linux-d_splice_alias-extra-iput

Even though we hope no one will actually ever need to use it, explain
this configure switch in the same place as the others.

Reviewed-on: http://gerrit.openafs.org/11749
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
(cherry picked from commit e02b852f05c0927d48b6467872378bae99df2760)

Change-Id: I4f7e42730afab2387d343880acc81d28b70ef5e9
Reviewed-on: http://gerrit.openafs.org/11752
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Tested-by: Daria Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>