Help: Move generator toolset support details to variable docs

Move the details about support for generator toolset specification
to the `CMAKE_GENERATOR_TOOLSET` variable documentation.  This is
a more suitable place because it is shared by all means to set this
variable, not just the `cmake -T` option.

Help/manual/OPTIONS_BUILD.txt

@@ -48,18 +48,12 @@
  build system.  Possible generator names are specified in the
  :manual:`cmake-generators(7)` manual.
 
-``-T <toolset-name>``
- Specify toolset name if supported by generator.
+``-T <toolset-spec>``
+ Toolset specification for the generator, if supported.
 
- Some CMake generators support a toolset name to be given to the
- native build system to choose a compiler.
- See the :variable:`CMAKE_GENERATOR_TOOLSET` variable.
- This is supported only on specific generators:
-
- * :ref:`Visual Studio Generators` for VS 2010 and above
- * The :generator:`Xcode` generator for Xcode 3.0 and above
-
- See native build system documentation for allowed toolset names.
+ Some CMake generators support a toolset specification to tell
+ the native build system how to choose a compiler.  See the
+ :variable:`CMAKE_GENERATOR_TOOLSET` variable for details.
 
 ``-A <platform-name>``
  Specify platform name if supported by generator.

Help/variable/CMAKE_GENERATOR_TOOLSET.rst

 CMAKE_GENERATOR_TOOLSET
 -----------------------
 
-Native build system toolset name specified by user.
+Native build system toolset specification provided by user.
 
-Some CMake generators support a toolset name to be given to the native
-build system to choose a compiler.  If the user specifies a toolset
-name (e.g.  via the :manual:`cmake(1)` ``-T`` option) the value will be
-available in this variable.
+Some CMake generators support a toolset specification to tell the
+native build system how to choose a compiler.  If the user specifies
+a toolset (e.g.  via the :manual:`cmake(1)` ``-T`` option) the value
+will be available in this variable.
 
 The value of this variable should never be modified by project code.
 A toolchain file specified by the :variable:`CMAKE_TOOLCHAIN_FILE`
 variable may initialize ``CMAKE_GENERATOR_TOOLSET``.  Once a given
 build tree has been initialized with a particular value for this
 variable, changing the value has undefined behavior.
+
+Toolset specification is supported only on specific generators:
+
+* :ref:`Visual Studio Generators` for VS 2010 and above
+* The :generator:`Xcode` generator for Xcode 3.0 and above
+
+See native build system documentation for allowed toolset names.