Help: Document cross compiling for Android

CMake now supports cross compiling for Android using the NDK or a
standalone toolchain.  Document the associated variables and how how to
write toolchain files for Android.

Help/variable/CMAKE_LANG_ANDROID_TOOLCHAIN_PREFIX.rst

@@ -0,0 +1,11 @@
+CMAKE_<LANG>_ANDROID_TOOLCHAIN_PREFIX
+-------------------------------------
+
+When :ref:`Cross Compiling for Android` this variable contains the absolute
+path prefixing the toolchain GNU compiler and its binutils.
+
+See also :variable:`CMAKE_<LANG>_ANDROID_TOOLCHAIN_SUFFIX`.
+
+For example, the path to the linker is::
+
+  ${CMAKE_CXX_ANDROID_TOOLCHAIN_PREFIX}ld${CMAKE_CXX_ANDROID_TOOLCHAIN_SUFFIX}