Docs should mention that cache KEY_FUNCTION can be a callable

[anonymous]

The cache docs currently state that:

• KEY_FUNCTION A string containing a dotted path to a function that defines how to compose a prefix, version and key into a final cache key.

However, it is also possible to specify a callable. I think this should be mentioned in the docs.

[arachnegl <gregoryloyse@…>]

In 8da86fd19b0907d3ea20593ca75073dce727d1aa:

Fixed #22398 -- Added detail about callable in KEY_FUNCTION docs

[Simon Charette <charettes@…>]

In 63d0cbab042c981d6ef63a3193fb1fa0027db0fd:

Merge pull request #2541 from arachnegl/22398_KEY_FUNCTION_doc_detail

Fixed #22398 -- Added detail about callable in KEY_FUNCTION docs

[Simon Charette <charette.s@…>]

In f004374f8ff0ad30a694a0f4b74ba24ca9cf3227:

[1.7.x] Fixed #22398 -- Added detail about callable in KEY_FUNCTION docs

Backport of 8da86fd19b from master