[Bug #1613651] Document socket.recv_into, socket.recvfrom_into
[Bug #1613651] Document socket.recv_into, socket.recvfrom_into

Also, the text for recvfrom told you to read recv() for an explanation of the
'flags' argument, but recv() just pointed you at the man page.  Copied the
man-page text to recvfrom(), recvfrom_into, recv_into to avoid the pointless
redirection.

I don't have LaTeX on this machine; hope my markup is OK.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator