test: clarify usage of tmpdir.refresh()

This emphasizes that `tmpdir.refresh()` must be called only once in each
test file when needed.

PR-URL: #37383
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

test/common/README.md

@@ -944,13 +944,18 @@ The realpath of the testing temporary directory.
 
 Deletes and recreates the testing temporary directory.
 
-The first time `refresh()` runs,  it adds a listener to process `'exit'` that
+The first time `refresh()` runs, it adds a listener to process `'exit'` that
 cleans the temporary directory. Thus, every file under `tmpdir.path` needs to
 be closed before the test completes. A good way to do this is to add a
 listener to process `'beforeExit'`. If a file needs to be left open until
 Node.js completes, use a child process and call `refresh()` only in the
 parent.
 
+It is usually only necessary to call `refresh()` once in a test file.
+Avoid calling it more than once in an asynchronous context as one call
+might refresh the temporary directory of a different context, causing
+the test to fail somewhat mysteriously.
+
 ## UDP pair helper
 
 The `common/udppair` module exports a function `makeUDPPair` and a class