Documentation: core-api: entry: Add comments about nesting

The topic of nesting and reentrancy in the context of early entry code
hasn't been addressed so far. So do it.

Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20220110105044.94423-2-nsaenzju@redhat.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>