Help: Add notes for topic 'GNUInstallDirs-special-prefixes'

Help/release/dev/GNUInstallDirs-special-prefixes.rst

+GNUInstallDirs-special-prefixes
+-------------------------------
+
+* The :module:`GNUInstallDirs` module learned special default values
+  for certain installation prefixes according to the `GNU Coding
+  Standards`_ and the `Filesystem Hierarchy Standard`_.
+
+.. _`GNU Coding Standards`: https://www.gnu.org/prep/standards/html_node/Directory-Variables.html
+.. _`Filesystem Hierarchy Standard`: https://refspecs.linuxfoundation.org/FHS_3.0/fhs/index.html