Help: Document CMAKE_<LANG>_STANDARD_LIBRARIES variable

Also drop documentation of non-existent CMAKE_STANDARD_LIBRARIES
variable.

Help/manual/cmake-variables.7.rst

@@ -68,7 +68,6 @@ Variables that Provide Information
    /variable/CMAKE_SKIP_INSTALL_RULES
    /variable/CMAKE_SKIP_RPATH
    /variable/CMAKE_SOURCE_DIR
-   /variable/CMAKE_STANDARD_LIBRARIES
    /variable/CMAKE_STATIC_LIBRARY_PREFIX
    /variable/CMAKE_STATIC_LIBRARY_SUFFIX
    /variable/CMAKE_TOOLCHAIN_FILE
@@ -366,6 +365,7 @@ Variables for Languages
    /variable/CMAKE_LANG_SIMULATE_VERSION
    /variable/CMAKE_LANG_SIZEOF_DATA_PTR
    /variable/CMAKE_LANG_SOURCE_FILE_EXTENSIONS
+   /variable/CMAKE_LANG_STANDARD_LIBRARIES
    /variable/CMAKE_USER_MAKE_RULES_OVERRIDE_LANG
 
 Variables for CTest

Help/release/dev/doc-standard-libs.rst

@@ -0,0 +1,6 @@
+doc-standard-libs
+-----------------
+
+* The :variable:`CMAKE_<LANG>_STANDARD_LIBRARIES` variable is now documented.
+  It is intended for use by toolchain files to specify system libraries to be
+  added to all linker command lines.

Help/variable/CMAKE_LANG_STANDARD_LIBRARIES.rst

@@ -0,0 +1,10 @@
+CMAKE_<LANG>_STANDARD_LIBRARIES
+-------------------------------
+
+Libraries linked into every executable and shared library linked
+for language ``<LANG>``.  This is meant for specification of system
+libraries needed by the language for the current platform.
+
+This variable should not be set by project code.  It is meant to be set by
+CMake's platform information modules for the current toolchain, or by a
+toolchain file when used with :variable:`CMAKE_TOOLCHAIN_FILE`.

Help/variable/CMAKE_STANDARD_LIBRARIES.rst

@@ -1,7 +0,0 @@
-CMAKE_STANDARD_LIBRARIES
-------------------------
-
-Libraries linked into every executable and shared library.
-
-This is the list of libraries that are linked into all executables and
-libraries.