Help: Note PROJECT_IS_TOP_LEVEL behavior w.r.t. ExternalProject

Extend the documentation added by commit 96a7040107 (project: Define
variables indicating whether project is top level, 2021-03-24,
v3.21.0-rc1~443^2) to give some examples of how the variables are set in
each context.

Help/variable/PROJECT-NAME_IS_TOP_LEVEL.rst

@@ -9,3 +9,13 @@ level ``CMakeLists.txt`` file.
 To obtain the value from the most recent call to :command:`project` in
 the current directory scope or above, see the
 :variable:`PROJECT_IS_TOP_LEVEL` variable.
+
+The variable value will be true in:
+
+* the top-level directory of the project
+* the top-level directory of an external project added by :module:`ExternalProject`
+
+The variable value will be false in:
+
+* a directory added by :command:`add_subdirectory`
+* a directory added by :module:`FetchContent`

Help/variable/PROJECT_IS_TOP_LEVEL.rst

@@ -19,3 +19,13 @@ options:
   if(PROJECT_IS_TOP_LEVEL)
     include(CTest)
   endif()
+
+The variable value will be true in:
+
+* the top-level directory of the project
+* the top-level directory of an external project added by :module:`ExternalProject`
+
+The variable value will be false in:
+
+* a directory added by :command:`add_subdirectory`
+* a directory added by :module:`FetchContent`