Document single-quoted strings more accurately

`'\a'` and `'\\a'` are both the same as `"\\a"` because single-quoted strings let `\` escape `\`.

(This fact makes it possible to write the string backslash+singlequote, `"\\'"`, as a single-quoted string `'\\\''`.)

What is the correct way in the .rdoc to escape the backslashes surrounded by `<tt>`? I'm not confident that I did *that* perfectly.  According to github preview, the last backslash kept trying to escape the `</tt>` instead of being a backslash character, so I sadly separated them with a space character.