Convert builtin help to reStructuredText source files

Run the convert-help.bash script to convert documentation:

 ./convert-help.bash "/path/to/CMake-build/bin"

Then remove it.

Help/variable/CMAKE_HOST_UNIX.rst

@@ -0,0 +1,7 @@
+CMAKE_HOST_UNIX
+---------------
+
+True for UNIX and UNIX like operating systems.
+
+Set to true when the host system is UNIX or UNIX like (i.e.  APPLE and
+CYGWIN).