Check*Source{Compiles,Runs}: Rewrite docs for these modules

Modules/CheckFortranSourceCompiles.cmake

@@ -1,35 +1,71 @@
 # Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
 # file Copyright.txt or https://cmake.org/licensing for details.
 
-#.rst:
-# CheckFortranSourceCompiles
-# --------------------------
-#
-# Check if given Fortran source compiles and links into an executable::
-#
-#   CHECK_Fortran_SOURCE_COMPILES(<code> <var> [FAIL_REGEX <fail-regex>]
-#                                 [SRC_EXT <ext>])
-#
-# The arguments are:
-#
-# ``<code>``
-#   Source code to try to compile.  It must define a PROGRAM entry point.
-# ``<var>``
-#   Variable to store whether the source code compiled.
-#   Will be created as an internal cache variable.
-# ``FAIL_REGEX <fail-regex>``
-#   Fail if test output matches this regex.
-# ``SRC_EXT <ext>``
-#   Use source extension ``.<ext>`` instead of the default ``.F``.
-#
-# The following variables may be set before calling this macro to modify
-# the way the check is run::
-#
-#   CMAKE_REQUIRED_FLAGS = string of compile command line flags
-#   CMAKE_REQUIRED_DEFINITIONS = list of macros to define (-DFOO=bar)
-#   CMAKE_REQUIRED_INCLUDES = list of include directories
-#   CMAKE_REQUIRED_LIBRARIES = list of libraries to link
-#   CMAKE_REQUIRED_QUIET = execute quietly without messages
+#[=======================================================================[.rst:
+CheckFortranSourceCompiles
+--------------------------
+
+Check if given Fortran source compiles and links into an executable.
+
+.. command:: check_fortran_source_compiles
+
+  ::
+
+    check_fortran_source_compiles(code resultVar
+        [FAIL_REGEX regex1 [regex2...]]
+        [SRC_EXT ext]
+    )
+
+  Check 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
+  variable specified by ``resultVar``, with a boolean true value for success
+  and boolean false for failure. If ``FAIL_REGEX`` is provided, then failure is
+  determined by checking if anything in the output matches any of the specified
+  regular expressions.
+
+  By default, the test source file will be given a ``.F`` file extension. The
+  ``SRC_EXT`` option can be used to override this with ``.ext`` instead.
+
+  The underlying check is performed by the :command:`try_compile` command. The
+  compile and link commands can be influenced by setting any of the following
+  variables prior to calling ``check_fortran_source_compiles()``:
+
+  ``CMAKE_REQUIRED_FLAGS``
+    Additional flags to pass to the compiler. Note that the contents of
+    :variable:`CMAKE_Fortran_FLAGS <CMAKE_<LANG>_FLAGS>` and its associated
+    configuration-specific variable are automatically added to the compiler
+    command before the contents of ``CMAKE_REQUIRED_FLAGS``.
+
+  ``CMAKE_REQUIRED_DEFINITIONS``
+    A :ref:`;-list <CMake Language Lists>` of compiler definitions of the form
+    ``-DFOO`` or ``-DFOO=bar``. A definition for the name specified by
+    ``resultVar`` will also be added automatically.
+
+  ``CMAKE_REQUIRED_INCLUDES``
+    A :ref:`;-list <CMake Language Lists>` of header search paths to pass to
+    the compiler. These will be the only header search paths used by
+    ``try_compile()``, i.e. the contents of the :prop_dir:`INCLUDE_DIRECTORIES`
+    directory property will be ignored.
+
+  ``CMAKE_REQUIRED_LIBRARIES``
+    A :ref:`;-list <CMake Language Lists>` of libraries to add to the link
+    command. These can be the name of system libraries or they can be
+    :ref:`Imported Targets <Imported Targets>` (see :command:`try_compile` for
+    further details).
+
+  ``CMAKE_REQUIRED_QUIET``
+    If this variable evaluates to a boolean true value, all status messages
+    associated with the check will be suppressed.
+
+  The check is only performed once, with the result cached in the variable
+  named by ``resultVar``. Every subsequent CMake run will re-use this cached
+  value rather than performing the check again, even if the ``code`` changes.
+  In order to force the check to be re-evaluated, the variable named by
+  ``resultVar`` must be manually removed from the cache.
+
+#]=======================================================================]
+
 
 macro(CHECK_Fortran_SOURCE_COMPILES SOURCE VAR)
   if(NOT DEFINED "${VAR}")