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/variable_watch.rst

@@ -0,0 +1,13 @@
+variable_watch
+--------------
+
+Watch the CMake variable for change.
+
+::
+
+  variable_watch(<variable name> [<command to execute>])
+
+If the specified variable changes, the message will be printed about
+the variable being changed.  If the command is specified, the command
+will be executed.  The command will receive the following arguments:
+COMMAND(<variable> <access> <value> <current list file> <stack>)