LUDOC-511 pool: reserved poolname "inherit" and "ignore"

LUDOC-511 pool: reserved poolname "inherit" and "ignore"

Add documentation for the special/reserved poolname "inherit" and
"ignore".

These reserved names were added by the patch:
https://review.whamcloud.com/46955 ("LU-15707 lod: force creation of a
component without a pool")

Change-Id: I411a810ed79c9d647d7d9a5e59066d33abb9ee21
Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/doc/manual/+/52712
Tested-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>