Doc updates for #apex and #systemapi

These include
- Drop the synonym sentence. With the enforcement introduced in
  aosp/2153678, they can no longer be used interchangeably
- Add information that a single .map.txt file cannot contain both these
  two annotations.
- Link a bug that provides additional context on why this granularity
  is necessary.

Test: N/A
Bug: 266824291
Change-Id: I31b49d492ec56f2f6f0b88de6e608e4ec6fe7011