Help: Document CMAKE_EXECUTABLE_SUFFIX_<LANG> explicitly

Documentation for `CMAKE_EXECUTABLE_SUFFIX_<LANG>`, useful when
cross-compiling, is buried in section `CMAKE_EXECUTABLE_SUFFIX`.

Help/manual/cmake-variables.7.rst

@@ -53,6 +53,7 @@ Variables that Provide Information
    /variable/CMAKE_DOTNET_TARGET_FRAMEWORK_VERSION
    /variable/CMAKE_EDIT_COMMAND
    /variable/CMAKE_EXECUTABLE_SUFFIX
+   /variable/CMAKE_EXECUTABLE_SUFFIX_LANG
    /variable/CMAKE_EXTRA_GENERATOR
    /variable/CMAKE_EXTRA_SHARED_LIBRARY_SUFFIXES
    /variable/CMAKE_FIND_DEBUG_MODE

Help/variable/CMAKE_EXECUTABLE_SUFFIX.rst

@@ -6,4 +6,5 @@ The suffix for executables on this platform.
 The suffix to use for the end of an executable filename if any, ``.exe``
 on Windows.
 
-``CMAKE_EXECUTABLE_SUFFIX_<LANG>`` overrides this for language ``<LANG>``.
+:variable:`CMAKE_EXECUTABLE_SUFFIX_<LANG>` overrides this for
+language ``<LANG>``.

Help/variable/CMAKE_EXECUTABLE_SUFFIX_LANG.rst

+CMAKE_EXECUTABLE_SUFFIX_<LANG>
+------------------------------
+
+The suffix to use for the end of an executable filename of ``<LANG>``
+compiler target architecture, if any.
+
+It overrides :variable:`CMAKE_EXECUTABLE_SUFFIX` for language ``<LANG>``.