Help: Move generator platform support details to variable docs

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

Help/manual/OPTIONS_BUILD.txt

@@ -66,13 +66,7 @@
 
  Some CMake generators support a platform name to be given to the
  native build system to choose a compiler or SDK.  See the
- :variable:`CMAKE_GENERATOR_PLATFORM` variable.
- This is supported only on specific generators:
-
- * For :ref:`Visual Studio Generators` with VS 2005 and above this
-   specifies the target architecture.
-
- See native build system documentation for allowed platform names.
+ :variable:`CMAKE_GENERATOR_PLATFORM` variable for details.
 
 ``-Wno-dev``
  Suppress developer warnings.

Help/variable/CMAKE_GENERATOR_PLATFORM.rst

 CMAKE_GENERATOR_PLATFORM
 ------------------------
 
-Generator-specific target platform name specified by user.
+Generator-specific target platform specification provided by user.
 
 Some CMake generators support a target platform name to be given
 to the native build system to choose a compiler toolchain.
@@ -13,3 +13,10 @@ A toolchain file specified by the :variable:`CMAKE_TOOLCHAIN_FILE`
 variable may initialize ``CMAKE_GENERATOR_PLATFORM``.  Once a given
 build tree has been initialized with a particular value for this
 variable, changing the value has undefined behavior.
+
+Platform specification is supported only on specific generators:
+
+* For :ref:`Visual Studio Generators` with VS 2005 and above this
+  specifies the target architecture.
+
+See native build system documentation for allowed platform names.