recv(2): update description of MSG_WAITALL

Remove a note about "data of a different type".  This was a bug that was
fixed in FreeBSD 15.  Instead put an exact quote from SUS that lists
allowed cases of a short read with MSG_WAITALL.  See discussion in D57511.

lib/libsys/recv.2

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd May 17, 2025
+.Dd September 6, 2026
 .Dt RECV 2
 .Os
 .Sh NAME
@@ -199,8 +199,10 @@ The
 flag requests that the operation block until
 the full request is satisfied.
 However, the call may still return less data than requested
-if a signal is caught, an error or disconnect occurs,
-or the next data to be received is of a different type than that returned.
+if the socket is a message-based socket, if a signal is caught,
+if the connection is terminated, if
+.Dv MSG_PEEK
+was specified, or if an error is pending for the socket.
 The
 .Dv MSG_DONTWAIT
 flag requests the call to return when it would block otherwise.