enable_testing: Update docs

- Described where this command shouldn't be called.
- Added examples section.
- Reworded few sentences.

Issue: #27564

Help/command/enable_testing.rst

@@ -1,20 +1,51 @@
 enable_testing
 --------------
 
-Enable testing for current directory and below.
+Enables testing for the current directory and below:
 
 .. code-block:: cmake
 
   enable_testing()
 
-Enables testing for this directory and below.
-
-This command should be in the top-level source directory because
+This command should be invoked in the top-level source directory because
 :manual:`ctest(1)` expects to find a test file in the top-level
 build directory.
 
-This command is automatically invoked when the :module:`CTest`
+This command is also automatically invoked when the :module:`CTest`
 module is included, except if the :variable:`BUILD_TESTING`
 option is turned off.
 
-See also the :command:`add_test` command.
+The following restrictions apply to where ``enable_testing()`` may be called:
+
+* It must be called in file scope, not in a :command:`function` call nor inside
+  a :command:`block`.
+
+Examples
+^^^^^^^^
+
+In the following example, this command is conditionally called depending on how
+the project is used.  For instance, when the Example project is added via the
+:module:`FetchContent` module as a subdirectory of a parent project that defines
+its own tests, testing for the Example project is disabled.
+
+.. code-block:: cmake
+  :caption: ``CMakeLists.txt``
+
+  project(Example)
+
+  option(Example_ENABLE_TESTING "Enable testing" ${PROJECT_IS_TOP_LEVEL})
+
+  if(Example_ENABLE_TESTING)
+    enable_testing()
+  endif()
+
+  # ...
+
+  if(Example_ENABLE_TESTING)
+    add_test(...)
+  endif()
+
+See Also
+^^^^^^^^
+
+* The :command:`add_test` command.