Help: Document section header underline hierarchy in cmake-developer.7

Explicitly specify the sequence of underline characters we use in the
CMake documentation.  It is the same sequence as that suggested in the
Sphinx documentation, but we have our own descriptions.

Help/manual/cmake-developer.7.rst

@@ -479,6 +479,15 @@ example:
 
 Capitalize the first letter of each non-minor word in the title.
 
+The section header underline character hierarchy is
+
+* ``#``: Manual group (part) in the master document
+* ``*``: Manual (chapter) title
+* ``=``: Section within a manual
+* ``-``: Subsection or `CMake Domain`_ object document title
+* ``^``: Subsubsection or `CMake Domain`_ object document section
+* ``"``: Paragraph or `CMake Domain`_ object document subsection
+
 Style: Whitespace
 ^^^^^^^^^^^^^^^^^