Unexpected character warning

[Kojoley]

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: