xprtrdma: Fix documenting comments in frwr_ops.c

Clean up.

FASTREG and LOCAL_INV WRs are typically not signaled. localinv_wake
is used for the last LOCAL_INV WR in a chain, which is always
signaled. The documenting comments should reflect that.

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