chore(docs): update and fix formatting gunicorn docs [backport 1.15] (#6079)

Backport 6c7aaad from #6060 to 1.15.

- Fixes the formatting for the following text: `ddtrace-run command and
it uses gevent`
<img width="860" alt="Screen Shot 2023-06-07 at 5 37 19 PM"
src="https://github.com/DataDog/dd-trace-py/assets/62392438/b5d6ab76-c373-4f3a-905d-651671b05c8d">

- Also instructs users to not use BOTH ``ddtrace-run`` and ``import
ddtrace.auto``. Importing `ddtrace.bootstrap.sitecustomize` twice raises
runtime errors and causes weird side effects.

## Checklist
- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/contributing.html#Release-Note-Guidelines)
are followed.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.

Co-authored-by: Munir Abdinur <[email protected]>

Author: github-actions[bot]

ddtrace/contrib/gunicorn/__init__.py

@@ -2,9 +2,10 @@
 ddtrace works with Gunicorn.
 
 .. note::
-    If you cannot wrap your Gunicorn server with the ``ddtrace-run``command and
-    it uses ``gevent`` workers, be sure to ``import ddtrace.auto`` as early as
+    If you cannot wrap your Gunicorn server with the ``ddtrace-run`` command and
+    it uses ``gevent`` workers be sure to ``import ddtrace.auto`` as early as
     possible in your application's lifecycle.
+    Do not use ``ddtrace-run`` with ``import ddtrace.auto``.
 """