assoc_array: correct the documentation of get_key_chunk()'s level argument

assoc_array.rst says the level passed to get_key_chunk() will be a
multiple of ASSOC_ARRAY_KEY_CHUNK_SIZE, and asks for the chunk starting
at that bit.  assoc_array_walk() after a shortcut, and both insertion
paths, pass the level of a node, which is any multiple of 4.  Each then
shifts the chunk it gets back right by level & ASSOC_ARRAY_KEY_CHUNK_MASK,
so what it wants is the chunk that contains that bit.

The keyring code copes, because keyring_get_key_chunk() divides the
level by ASSOC_ARRAY_KEY_CHUNK_SIZE before using it.  An implementation
written to the documentation does not: one that returns a one-word key
for level 0 and nothing for any other level files leaves under the
wrong slot the first time a node is split, and a later insertion hits
the BUG_ON() after found_slot_for_multiple_occupancy in
assoc_array_insert_into_terminal_node().

Say what the code expects.  get_object_key_chunk() is documented as
"the previous function", so this covers it too.

Assisted-by: LLM
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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