bug#64537: mix lists and vectors when using backquote

[Shynur Xie]

The docstring of ‘backquote’ says “vectors work just like lists”.
Should we also clarify that mixing lists and vectors these 2 way:

    `(... ,@[...])  ; by contrast: `(,@[] 1)  => (1)
    `[,@'()]        ; by contrast: `[1 ,@'()] => [1]

will lead to unexpected behavior?  Or suggest not doing so?  Or let
‘backquote’ detect this and signal an error?

BTW, if [[https://debbugs.gnu.org/cgi/bugreport.cgi?bug=64415]] is
actually not a bug, please consider document that behavior.  Thanks.