Clarify enumerate docs: after and maxWaitSec are mutually exclusive

doc/protocol/blob-enumerate-protocol.txt

@@ -14,6 +14,8 @@ URL GET parameters:
      after     optional    If provided, only blobs GREATER THAN this
                            value are returned.
 
+                           Can't be used in combination with 'maxwaitsec'
+
      limit     optional    Limit the number of returned blobrefs.  The
                            server may have its own lower limit, however,
                            so be sure to pay attention to the presence
@@ -35,6 +37,8 @@ URL GET parameters:
                            'maxwaitsec' and wait for less time than
                            requested by the client.
 
+                           Can't be used in combination with 'after'.
+
 
 Response:
 

lib/go/camli/blobserver/interface.go

@@ -83,6 +83,9 @@ type BlobEnumerator interface {
 	// or 0 for no delay.
 	// EnumerateBlobs must close the channel.  (even if limit
 	// was hit and more blobs remain)
+	//
+	// after and waitSeconds can't be used together. One must be
+	// its zero value.
 	EnumerateBlobs(dest chan<- blobref.SizedBlobRef,
 	after string,
 	limit uint,