xprtrdma: Replace outdated comment for rpcrdma_ep_post

Since commit 7c8d9e7c

 ("xprtrdma: Move Receive posting to
Receive handler"), rpcrdma_ep_post is no longer responsible for
posting Receive buffers. Update the documenting comment to reflect
this change.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>