Help: Document the CMAKE_TOOLCHAIN_FILE.

Link to the new docs from existing references to the variable.

Help/manual/cmake-variables.7.rst

@@ -64,6 +64,7 @@ Variables that Provide Information
    /variable/CMAKE_STANDARD_LIBRARIES
    /variable/CMAKE_STATIC_LIBRARY_PREFIX
    /variable/CMAKE_STATIC_LIBRARY_SUFFIX
+   /variable/CMAKE_TOOLCHAIN_FILE
    /variable/CMAKE_TWEAK_VERSION
    /variable/CMAKE_VERBOSE_MAKEFILE
    /variable/CMAKE_VERSION

Help/variable/CMAKE_LANG_COMPILER_EXTERNAL_TOOLCHAIN.rst

@@ -10,4 +10,4 @@ may be set to a path to a path to the external toolchain and will be passed
 to the compiler driver if supported.
 
 This variable may only be set in a toolchain file specified by
-the ``CMAKE_TOOLCHAIN_FILE`` variable.
+the :variable:`CMAKE_TOOLCHAIN_FILE` variable.

Help/variable/CMAKE_LANG_COMPILER_TARGET.rst

@@ -8,4 +8,4 @@ QNX qcc. These compiler drivers support a command-line argument to specify
 the target to cross-compile for.
 
 This variable may only be set in a toolchain file specified by
-the ``CMAKE_TOOLCHAIN_FILE`` variable.
+the :variable:`CMAKE_TOOLCHAIN_FILE` variable.

Help/variable/CMAKE_SYSROOT.rst

@@ -9,4 +9,4 @@ necessary on installation.  The ``CMAKE_SYSROOT`` is also used to prefix
 paths searched by the ``find_*`` commands.
 
 This variable may only be set in a toolchain file specified by
-the ``CMAKE_TOOLCHAIN_FILE`` variable.
+the :variable:`CMAKE_TOOLCHAIN_FILE` variable.

Help/variable/CMAKE_TOOLCHAIN_FILE.rst

@@ -0,0 +1,9 @@
+CMAKE_TOOLCHAIN_FILE
+--------------------
+
+Path to toolchain file supplied to :manual:`cmake(1)`.
+
+This variable is specified on the command line when cross-compiling with CMake.
+It is the path to a file which is read early in the CMake run and which specifies
+locations for compilers and toolchain utilities, and other target platform and
+compiler related information.