Fix usage of reST syntax in prefix parameter of meta

[sleiner]

The usage of the prefix parameter of the meta layout function (as per the docs) does not work. There are two problems with this:

1. The documentation for meta does not mention that backslashes need to be escaped and shows unescaped backslashes instead. The docs for meta_all do not make this mistake.
2. Since version 0.16, docutils inserts null-characters when parsing escaped backslashes, which in turn is not accepted by ast.parse. The solution is described in the release notes for docutils 0.16: Using node.astext() instead of str(node)

[danwos]

Thanks for the PR and the updated docs 👍

If you like, you can also add yourself to the AUTHORS list.
I will merge this PR tomorrow, so hurry up ;) (Or create a second PR for the AUTHORS update later)