Fix documentation of requestCode parameter.

It's not correct to state requestCode is 'currently not used'. It can be
used to differentiate multiple PendingIntents since a number of Android
versions now. Also, the introduction block of the PendingIntent
documentation explicitly states this as a possible use:
"If you truly need multiple distinct PendingIntent objects active at the same
time [...], then you will need to ensure there is something that is different
about them to associate them with different PendingIntents. This may be
any of the Intent attributes considered by Intent.filterEquals, or
different request code integers supplied [...]"

So fix the documentation inconsistency to make the possible use
immediately clear.

Change-Id: Ia65d7a5ab12c2c3ee1aa1c5107e5ea8fd937b765