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)

Modules/CheckIncludeFileCXX.cmake

@@ -35,8 +35,24 @@ the way the check is run:
 
 .. include:: /module/CMAKE_REQUIRED_QUIET.txt
 
-See modules :module:`CheckIncludeFile` and :module:`CheckIncludeFiles`
-to check for one or more ``C`` headers.
+Examples
+^^^^^^^^
+
+Checking whether the ``C++23`` header ``<stdfloat>`` exists and storing the
+check result in the ``HAVE_STDFLOAT_HEADER`` cache variable:
+
+.. code-block:: cmake
+
+  include(CheckIncludeFileCXX)
+
+  check_include_file_cxx(stdfloat HAVE_STDFLOAT_HEADER)
+
+See Also
+^^^^^^^^
+
+* The :module:`CheckIncludeFile` 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)

Modules/CheckIncludeFiles.cmake

@@ -41,8 +41,41 @@ the way the check is run:
 
 .. include:: /module/CMAKE_REQUIRED_QUIET.txt
 
-See modules :module:`CheckIncludeFile` and :module:`CheckIncludeFileCXX`
-to check for a single header file in ``C`` or ``CXX`` languages.
+Examples
+^^^^^^^^
+
+Checking whether one or more ``C`` headers exist and storing the check result
+in cache variables:
+
+.. code-block:: cmake
+
+  include(CheckIncludeFiles)
+
+  check_include_files(sys/socket.h HAVE_SYS_SOCKET_H)
+
+  if(HAVE_SYS_SOCKET_H)
+    # The <net/if.h> header on Darwin and BSD-like systems is not self-contained
+    # and also requires <sys/socket.h>
+    check_include_files("sys/socket.h;net/if.h" HAVE_NET_IF_H)
+  else()
+    check_include_files(net/if.h HAVE_NET_IF_H)
+  endif()
+
+The ``LANGUAGE`` option can be used to specify which compiler to use.  For
+example, checking multiple ``C++`` headers, when both ``C`` and ``CXX``
+languages are enabled in the project:
+
+.. code-block:: cmake
+
+  include(CheckIncludeFiles)
+
+  check_include_files("header_1.hpp;header_2.hpp" HAVE_HEADERS LANGUAGE CXX)
+
+See Also
+^^^^^^^^
+
+* The :module:`CheckIncludeFile` module to check for single ``C`` header.
+* The :module:`CheckIncludeFileCXX` module to check for single ``C++`` header.
 #]=======================================================================]
 
 include_guard(GLOBAL)