Improve docs around "on demand time" capability

Update the docs to record how the behavior differs from the name.

current.txt has been updated by hand using information from:
hidl-gen -L hash -r android.hardware:hardware/interfaces \
  android.hardware.gnss@1.0::IGnssCallback

As per instructions at
https://source.android.com/docs/core/architecture/hidl/hashing

This is acceptable because it's just a docs change that documents
existing, historic behavior. Previous docs were misleading.

Test: hidl-gen -L check -r android.hardware:hardware/interfaces android.hardware.gnss@1.0::IGnssCallback
Bug: 222295093
Bug: 73893222
Change-Id: I935354be52239482d559962e9a9ee1faaf73f4b1