dt-bindings: power: sbs-battery: re-document "ti,bq20z75"

This compatible property was documented before the driver was renamed to
"SBS" (see commit e57f1b68 ("devicetree-bindings: Propagate
bq20z75->sbs rename to dt bindings")). The driver has continued to
support this property as an alternative to "sbs,sbs-battery", and
because we've noticed there are some lingering TI specifics (in the
manufacturer-specific portion of the SBS spec), we'd like to start using
this property again to differentiate.

In typical DT fashion, the <vendor>,<part-number> specifics should be
used ahead of the generic "sbs,sbs-battery" string, so we can handle
vendor specifics -- so document this. Language borrowed mostly from
Documentation/devicetree/bindings/power/supply/sbs_sbs-charger.txt

Also fixup the example to use this property (it's already implying that
it's "bq20z75@b"); fixup the node name to be generic ("battery", not
"<part-number>"); and fixup some whitespace.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Acked-by: Rhyland Klein <rklein@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>