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.