doc: Update INSTALL w/info on overriding dirpaths

doc: Update INSTALL w/info on overriding dirpaths

Fix a typo in INSTALL for 'afslogsdir'.

Add a note with information that it is possible to override the
directory paths via environment variables.

The added documentation is targeting distro maintainers so they are
aware that the directory paths can be configured without patching.

Change-Id: Ia94d8525db798fe01866126f1f315499296717c9
Reviewed-on: https://gerrit.openafs.org/14888
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Ralf Brunckhorst <rbrunckhorst@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>