Document the limitation to a hundred callbacks.

Some developers have been surprised by this limitation and had trouble
figuring out what the issue was. Add documentation to address this.

This also includes a drive-by removal of a duplicate check.

Bug: 149867479
Test: doc-only change
Original-Change: https://android-review.googlesource.com/1313813
Merged-In: I5911d01984695550b6c9afe7a8eb535bf5e320a1
Change-Id: I5911d01984695550b6c9afe7a8eb535bf5e320a1