Document MemPoolAccept::Finalize(...) precondition.
Context: A quick look at ...
$ git grep m_conflicting_size
src/validation.cpp: size_t m_conflicting_size;
src/validation.cpp: size_t& nConflictingSize = ws.m_conflicting_size;
src/validation.cpp: const size_t& nConflictingSize = ws.m_conflicting_size;
... might look like an uninitialized read (which it is not) :)