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
Change-Id: I5911d01984695550b6c9afe7a8eb535bf5e320a1
3 files changed
tree: 2849abf96a3d05ee67f686f28d28e6615764e643
  1. core/
  2. services/
  3. tests/