CheckInclude{FileCXX,File,Files}: Add examples

This adds some examples to modules for checking headers.

Modules/CheckIncludeFile.cmake

@@ -35,9 +35,24 @@ the way the check is run:
 
 .. include:: /module/CMAKE_REQUIRED_QUIET.txt
 
-See the :module:`CheckIncludeFiles` module to check for multiple headers
-at once.  See the :module:`CheckIncludeFileCXX` module to check for headers
-using the ``CXX`` language.
+Examples
+^^^^^^^^
+
+Checking whether the ``C`` header ``<unistd.h>`` exists and storing the check
+result in the ``HAVE_UNISTD_H`` cache variable:
+
+.. code-block:: cmake
+
+  include(CheckIncludeFile)
+
+  check_include_file(unistd.h HAVE_UNISTD_H)
+
+See Also
+^^^^^^^^
+
+* The :module:`CheckIncludeFileCXX` module to check for single ``C++`` header.
+* The :module:`CheckIncludeFiles` module to check for one or more ``C`` or
+  ``C++`` headers at once.
 #]=======================================================================]
 
 include_guard(GLOBAL)