docs: document that fusermount3 may be needed when mounting/unmounting

See: https://forum.rclone.org/t/documentation-fusermount-vs-fusermount3/47816/

Author: ncw

cmd/mountlib/mount.md

@@ -42,7 +42,9 @@ When running in background mode the user will have to stop the mount manually:
 
     # Linux
     fusermount -u /path/to/local/mount
-    # OS X
+    #... or on some systems
+    fusermount3 -u /path/to/local/mount
+    # OS X or Linux when using nfsmount
     umount /path/to/local/mount
 
 The umount operation can fail, for example when the mountpoint is busy.
@@ -386,9 +388,9 @@ Note that systemd runs mount units without any environment variables including
 `PATH` or `HOME`. This means that tilde (`~`) expansion will not work
 and you should provide `--config` and `--cache-dir` explicitly as absolute
 paths via rclone arguments.
-Since mounting requires the `fusermount` program, rclone will use the fallback
-PATH of `/bin:/usr/bin` in this scenario. Please ensure that `fusermount`
-is present on this PATH.
+Since mounting requires the `fusermount` or `fusermount3` program,
+rclone will use the fallback PATH of `/bin:/usr/bin` in this scenario.
+Please ensure that `fusermount`/`fusermount3` is present on this PATH.
 
 ### Rclone as Unix mount helper