doc/develop/sending_patches.rst: Clarify when to use which branch

The previous wording on the paragraph about what branch to use when
submitting patches did not reflect how / when the next branch is
currently used. Reword this to note that master should be used for bug
and regression fixes, always, and that next should be used once it
opens, with -rc2.

Reported-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>