Unexpected character warning

I am trying to fix Spirit docs https://github.com/boostorg/spirit/blob/develop/doc/lex/lexer.qbk#L34, but having troubles with bogus warning or broken escaping.

Given:
```
[[`"[xyz]\"foo"`] [The literal string `[xyz]"foo`]]
```

Renders correctly:

But generates a warning:
```
lex/lexer.qbk:34: warning: in column:50, unexpected character: "
```

Fixing it like this:
```
[[`"[xyz]\"foo"`] [The literal string `[xyz]\"foo`]]
```

Does not eliminate it:
```
lex/lexer.qbk:34: warning: in column:51, unexpected character: "
```

And leads to incorrect rendering: