Warn about --basetemp removing the entire directory (#7555)

Co-authored-by: mattreex <mattreex.9@gail.com>
Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>
(cherry picked from commit 1e66ed0)

doc/en/tmpdir.rst

@@ -192,8 +192,13 @@ You can override the default temporary directory setting like this:
 
     pytest --basetemp=mydir
 
-When distributing tests on the local machine, ``pytest`` takes care to
-configure a basetemp directory for the sub processes such that all temporary
+.. warning::
+
+    The contents of ``mydir`` will be completely removed, so make sure to use a directory
+    for that purpose only.
+
+When distributing tests on the local machine using ``pytest-xdist``, care is taken to
+automatically configure a basetemp directory for the sub processes such that all temporary
 data lands below a single per-test run basetemp directory.
 
 .. _`py.path.local`: https://py.readthedocs.io/en/latest/path.html