fs: add a kerneldoc header over lookup_fast

The lookup_fast helper in fs/namei.c has some subtlety in how dentries
are returned. Document them.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20240802-openfast-v1-2-a1cff2a33063@kernel.org


Signed-off-by: Christian Brauner <brauner@kernel.org>