Check{,C,CXX,OBJC,OBJCXX,Fortran}SourceCompiles: Update documentation

Changes:

- Added intro code blocks showing how to include these modules.
- Added examples sections.
- Used "command" instead of "macro".
- Commands sections added to have a clearer overview of the modules at
  first encounter.
- Reworded commands arguments a bit.
- Added a rubric title for variables that affect the checks.
- CheckFortranSourceCompiles: Added CMake version when SRC_EXT option
  was introduced.
- Listed CMAKE_TRY_COMPILE_TARGET_TYPE variable in the rubric together
  with CMAKE_REQUIRED_* variables and added an include RST file for it.
- Used lowercase style for check_fortran_source_compiles().

Help/guide/tutorial/Adding System Introspection.rst

@@ -35,7 +35,7 @@ exercise, complete ``TODO 1`` through ``TODO 5``.
 
 Start by editing ``MathFunctions/CMakeLists.txt``. Include the
 :module:`CheckCXXSourceCompiles` module. Then, use
-``check_cxx_source_compiles`` to determine whether ``log`` and ``exp`` are
+``check_cxx_source_compiles()`` to determine whether ``log`` and ``exp`` are
 available from ``cmath``. If they are available, use
 :command:`target_compile_definitions` to specify ``HAVE_LOG`` and ``HAVE_EXP``
 as compile definitions.

Help/module/include/CMAKE_TRY_COMPILE_TARGET_TYPE.rst

@@ -0,0 +1,6 @@
+:variable:`CMAKE_TRY_COMPILE_TARGET_TYPE`
+  Internally, the :command:`try_compile` command is used to perform the
+  check, and this variable controls the type of target it creates.  If this
+  variable is set to ``EXECUTABLE`` (the default), the check compiles and
+  links the test source code as an executable program.  If set to
+  ``STATIC_LIBRARY``, the test source code is compiled but not linked.

Help/release/3.1.rst

@@ -191,7 +191,7 @@ Modules
   meant for a prefix other than :variable:`CMAKE_INSTALL_PREFIX`.
 
 * The :module:`CheckFortranSourceCompiles` module was added to
-  provide a ``CHECK_Fortran_SOURCE_COMPILES`` macro.
+  provide a ``check_fortran_source_compiles()`` command.
 
 * The :module:`ExternalData` module learned to tolerate a ``DATA{}``
   reference to a missing source file with a warning instead of

Help/release/3.7.rst

@@ -158,8 +158,8 @@ Modules
 * An :module:`AndroidTestUtilities` module was added to manage transfer
   of test data to an Android device.
 
-* The :module:`CheckFortranSourceCompiles` module macro
-  ``CHECK_Fortran_SOURCE_COMPILES`` gained a ``SRC_EXT`` option
+* The :module:`CheckFortranSourceCompiles` module command
+  ``check_fortran_source_compiles()`` gained a ``SRC_EXT`` option
   to specify a custom test Fortran source file extension.
 
 * The :module:`ExternalProject` module gained ``HTTP_USERNAME`` and