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/command/return.rst

@@ -0,0 +1,18 @@
+return
+------
+
+Return from a file, directory or function.
+
+::
+
+  return()
+
+Returns from a file, directory or function.  When this command is
+encountered in an included file (via include() or find_package()), it
+causes processing of the current file to stop and control is returned
+to the including file.  If it is encountered in a file which is not
+included by another file, e.g.  a CMakeLists.txt, control is returned
+to the parent directory if there is one.  If return is called in a
+function, control is returned to the caller of the function.  Note
+that a macro is not a function and does not handle return like a
+function does.