zephyr: Document pwr_avg cmd in batteries doc
zephyr: Document pwr_avg cmd in batteries doc

The pwr_avg EC console command can be invoked to query battery charging
rates.

Document this functionality in the Zephyr battery doc.

BRANCH=none
BUG=b:216366887
TEST=gerrit view

Signed-off-by: Aaron Massey <79529a820df71f45023239a3663736629646cc34@google.com>
Change-Id: I139aa8d83dcdc0d80067b95fafbca77c48e3fdaa
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3576667
Reviewed-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec