More docs for LowPowerStandbyPolicy, rename FEATURE_WAKE_ON_LAN

Rename LOW_POWER_STANDBY_FEATURE_WAKE_ON_LAN to
FEATURE_WAKE_ON_LAN_IN_LOW_POWER_STANDBY and better document how it is
used in LowPowerStandbyPolicy.

Bug: 269254783
Test: m
Change-Id: I40c547e78c5f123eb6bcc9f365b46dcb994cae6b