regulator: Fix regulator_get_{optional,exclusive}() documentation

regulator_get_optional() doesn't hold an exclusive reference to
the regulator. Fix the documentation and reword the exclusive
documentation to fix the grammatical error "this reference is
held".

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Mark Brown <broonie@linaro.org>