clk: Improve documentation for devm_clk_get() and its optional variant

clk: Improve documentation for devm_clk_get() and its optional variant

Make use of "Context:" and "Return:". Mention that the clk is not to be
expected to be prepared, previously only not being enabled was mentioned
which probably dates from the times when the concept of clk preparation
wasn't invented yet.

Also describe devm_clk_get_optional() fully instead of just referencing
devm_clk_get().

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20220520075737.758761-2-u.kleine-koenig@pengutronix.de
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>