docs/core-api: memory-allocation: GFP_NOWAIT doesn't need __GFP_NOWARN

Since v6.8 the definition of GFP_NOWAIT has implied __GFP_NOWARN,
so it is now redundant to add this flag explicitly.

Update the docs to match, and emphasise the need for a fallback
when using GFP_NOWAIT.

Fixes: 16f5dfbc ("gfp: include __GFP_NOWARN in GFP_NOWAIT")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240729140127.244606-1-Dave.Martin@arm.com