docs: power: clean up power_supply_class.rst

docs: power: clean up power_supply_class.rst

Clean up grammar, punctuation, etc., in the power supply class
documentation.

Add article adjectives where needed.
Hyphenate some adjectives.
Fix punctuation.
Fix some verb usage (singular/plural).
Fix run-on sentences.
Add "is" in a few places.
Change "QA" to "Q&A".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Link: https://patch.msgid.link/20251014182008.823980-1-rdunlap@infradead.org
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>