docs/,net/Makefile.mk,net/remote_protocol.rst,
net/replication_protocol.rst: Move the protocol docs for the remote
and replication protocols into the net/ subdirectory.

git-svn-id: file:///home/xapian-svn/xapian/trunk@18314 6b2f1b92-a4f3-0310-943f-de3cbe41748b