power: reset: ltc2952: document optional trigger behavior

Document the fact that the trigger signal is now optional, and describe
the behavior when this is used.

While at it, fix a typo, and paraphrase a sentence to be less platform
specific.

Signed-off-by: Frans Klaver <frans.klaver@xsens.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>