* Some clarifications to section on virtual fns
Primarily, I made it clear that PYBIND11_OVERLOAD_PURE_NAME is not "useful" but required in renaming situations. Also clarified that one should not bind to the trampoline helper class which I found tempting since it seems more explicit.
* Remove :emphasize-lines: from cpp block, seems to suppress formatting
* docs: emphasize default policy, clarify keep_alive
Emphasize the default return value policy since this statement is hidden in a wall of text.
Add a hint that call policies are probably required for container objects.
