[Debugger] Clarify that only one paused task can be debugged at a time (

ray-project#48415)

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## Why are these changes needed?

I was initially confused that I couldn't join another paused task while
a debugger was in "continue" mode.

## Related issue number

<!-- For example: "Closes ray-project#1234" -->

## Checks

- [ ] I've signed off every commit(by using the -s flag, i.e., `git
commit -s`) in this PR.
- [ ] I've run `scripts/format.sh` to lint the changes in this PR.
- [ ] I've included any doc changes needed for
https://docs.ray.io/en/master/.
- [ ] I've added any new APIs to the API Reference. For example, if I
added a
method in Tune, I've added it in `doc/source/tune/api/` under the
           corresponding `.rst` file.
- [ ] I've made sure the tests are passing. Note that there might be a
few flaky tests, see the recent failures at https://flakey-tests.ray.io/
- Testing Strategy
   - [ ] Unit tests
   - [ ] Release tests
   - [ ] This PR is not tested :(

---------

Signed-off-by: Philipp Moritz <[email protected]>
Co-authored-by: bhuang <[email protected]>
Co-authored-by: angelinalg <[email protected]>

doc/source/ray-observability/ray-distributed-debugger.rst

@@ -77,10 +77,14 @@ When the debugger hits a breakpoint:
     :align: center
 
 
-Start debugging
-~~~~~~~~~~~~~~~
+Start and stop debugging
+~~~~~~~~~~~~~~~~~~~~~~~~
 
-Debug your Ray app as you would when developing locally.
+Debug your Ray app as you would when developing locally. After you're done debugging this particular
+breakpoint, click the **Disconnect** button in the debugging toolbar so you can join another task
+in the **Paused Tasks** list.
+
+.. figure:: ./images/debugger-disconnect.gif
 
 
 Post-mortem debugging