docs: Clarify wording on two Javadoc entries

Per feedback from dmail@, rephrased to clarify that the intent is
fired at the time when a USB device/accessory is attached, not a
general "a USB device happens to be attached now" notification.

Staged to:

http://go/dac-stage/reference/android/hardware/usb/UsbManager.html

Bug: 36672976
Test: make ds-docs
Change-Id: I9f1c202d0a6a6a5b25cf775e2b11119a26625bf1