gh-155717: Reword the NEWS entry for the default start method change (GH-159273)

Say that the change applies when there is no writable temporary
directory and that starting a process previously failed there.

Author: gpshead

Misc/NEWS.d/next/Library/2026-08-15-09-47-23.gh-issue-155717.jWFLR2.rst

@@ -1,3 +1,5 @@
-:mod:`multiprocessing`'s default start method on systems with non-writeable
-tempfile filesystem is now :ref:`"spawn" <multiprocessing-start-methods>`
-instead of ``"forkserver"``. Patch by Bénédikt Tran.
+:mod:`multiprocessing`'s default start method is now
+:ref:`"spawn" <multiprocessing-start-methods>` instead of ``"forkserver"``
+when there is no writable temporary directory, such as on a read-only file
+system. Previously starting a process failed with an :exc:`OSError` there.
+Patch by Bénédikt Tran.