Add docs for remaining charge time sensor

[dshokouhi]

docs for: https://github.com/home-assistant/android/pull/4346

[jpelgrom]

This should mention the unit of measurement (ms)

[dshokouhi]

This should mention the unit of measurement (ms)

thanks for calling it out, updated to minutes per latest PR changes and also mentioning unavailable and 0 states as users may find 0 to be misleading when its a valid value from the API

[jpelgrom]

Now this claims 0 and unavailable are the same, whereas the description on the Android PR says they're (subtly) different...