Help: Clarify CMAKE_VERBOSE_MAKEFILE initialization (#14658)

Explain that it is initialized to false by the project() command and
intended for configuration by the end user performing a build.

Help/variable/CMAKE_VERBOSE_MAKEFILE.rst

@@ -1,8 +1,9 @@
 CMAKE_VERBOSE_MAKEFILE
 ----------------------
 
-Create verbose makefiles if on.
+Enable verbose output from Makefile builds.
 
-This variable defaults to false.  You can set this variable to true to
-make CMake produce verbose makefiles that show each command line as it
-is used.
+This variable is a cache entry initialized (to FALSE) by
+the :command:`project` command.  Users may enable the option
+in their local build tree to get more verbose output from
+Makefile builds and show each command line as it is launched.