Document that PassManager::add() may delete the pass right away.

Also remove redundant documentation:
- doxygen will copy documentation to overriden methods.
- Use \copydoc on PIMPL classes instead of replicating the text.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224089 91177308-0d34-0410-b5e6-96231b3b80d8