regcache: Improve documentation of available cache types

There is some user confusion about which cache types to choose when which
is not helped by the lack of any central documentation providing an
overview of what's available. Provide a short overview in the API header to
try to help reduce this.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://patch.msgid.link/20240924-regcache-document-types-v1-1-e157054e1215@kernel.org


Signed-off-by: Mark Brown <broonie@kernel.org>