LUDOC-522 Nodemap: Added nodemap_del examples.

LUDOC-522 Nodemap: Added nodemap_del examples.

The Lustre manual was missing expamples for nodemap_del and
nodemap_del_range. Added a section with examples for both that
will fit the existing style and documentation.

Signed-off-by: Maximilian Dilger <mdilger@whamcloud.com>
Change-Id: I56630e10941972099871bc6adad2558a4182d01f
Reviewed-on: https://review.whamcloud.com/c/doc/manual/+/55368
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>