Help: Clarify VS_STARTUP_PROJECT documentation

Explain in what directory the property should be set to affect the
corresponding ``.sln`` file.

Suggested-by: Stephen Kelly <[email protected]>

Help/prop_dir/VS_STARTUP_PROJECT.rst

@@ -3,10 +3,16 @@ VS_STARTUP_PROJECT
 
 Specify the default startup project in a Visual Studio solution.
 
+The :ref:`Visual Studio Generators` create a ``.sln`` file for each directory
+whose ``CMakeLists.txt`` file calls the :command:`project` command.  Set this
+property in the same directory as a :command:`project` command call (e.g. in
+the top-level ``CMakeLists.txt`` file) to specify the default startup project
+for the correpsonding solution file.
+
 The property must be set to the name of an existing target.  This
 will cause that project to be listed first in the generated solution
 file causing Visual Studio to make it the startup project if the
 solution has never been opened before.
 
-If this property is not specified, then the "ALL_BUILD" project
+If this property is not specified, then the ``ALL_BUILD`` project
 will be the default.