Revert "Documentation: NFS/RDMA: Document separate Kconfig symbols"

This reverts commit 731d5cca.

Commit ffe1f0df

 ("rpcrdma: Merge svcrdma and xprtrdma modules into
one") forgot to update the corresponding documentation.

Reported-by: Valentin Rothberg <valentinrothberg@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>