clarity of @xref warning

[Kevin Ryde]

The incorrect @xref usage in foo.texi below provokes

        foo.texi:2: warning: `.' or `,' must follow @xref, not `)'.

For a long time I thought this meant the "*Note" would come out
invalid, or something.  Perhaps the message could be clarified to say
why this form is wrong, maybe something like

        foo.texi:2: warning, @xref followed by `)'.
            @xref is meant for use as a whole sentence, or the start of
            one, not in brackets "( )".  Perhaps you mean @pxref.

foo.texi
Description: TeXInfo document