Help: Add examples to CheckFortranSource{Runs,Compiles}

Modules/CheckFortranSourceCompiles.cmake

@@ -17,8 +17,20 @@ Check if given Fortran source compiles and links into an executable.
     )
 
   Checks that the source supplied in ``<code>`` can be compiled as a Fortran
-  source file and linked as an executable (so it must contain at least a
-  ``PROGRAM`` entry point). The result will be stored in the internal cache
+  source file and linked as an executable. The ``<code>`` must be a Fortran program
+  containing at least an ``end`` statement--for example:
+
+  .. code-block:: cmake
+
+    check_fortran_source_compiles("character :: b; error stop b; end" F2018ESTOPOK SRC_EXT F90)
+
+  This command can help avoid costly build processes when a compiler lacks support
+  for a necessary feature, or a particular vendor library is not compatible with
+  the Fortran compiler version being used. This generate-time check may advise the
+  user of such before the main build process. See also the
+  :command:`check_fortran_source_runs` command to actually run the compiled code.
+
+  The result will be stored in the internal cache
   variable ``<resultVar>``, with a boolean true value for success and boolean
   false for failure.
 

Modules/CheckFortranSourceRuns.cmake

@@ -16,8 +16,20 @@ subsequently be run.
         [SRC_EXT <extension>])
 
   Check that the source supplied in ``<code>`` can be compiled as a Fortran source
-  file, linked as an executable and then run. The ``<code>`` must contain at
-  least ``program; end program`` statements. If the ``<code>`` could be built and run
+  file, linked as an executable and then run. The ``<code>`` must be a Fortran program
+  containing at least an ``end`` statement--for example:
+
+  .. code-block:: cmake
+
+    check_fortran_source_runs("real :: x[*]; call co_sum(x); end" F2018coarrayOK)
+
+  This command can help avoid costly build processes when a compiler lacks support
+  for a necessary feature, or a particular vendor library is not compatible with
+  the Fortran compiler version being used. Some of these failures only occur at runtime
+  instead of linktime, and a trivial runtime example can catch the issue before the
+  main build process.
+
+  If the ``<code>`` could be built and run
   successfully, the internal cache variable specified by ``<resultVar>`` will
   be set to 1, otherwise it will be set to an value that evaluates to boolean
   false (e.g. an empty string or an error message).