Document Escape Sequences

Escape sequences are neither mentioned nor listed in the docs, so when I went to check what needs to be escape (if anything) there was nothing to see, which was disorienting since they do exist.

I'm not sure what all of them are, besides `\\` and `\"`

No documentation file, test, or entry point is named in the issue. Start by locating the documentation for string escaping and identify the escape sequences supported by HCL; done means that documentation lists them clearly, including the examples mentioned in the issue.

Written by the indexing model from the issue text.