PR93253 – Document BOZ changes, make it friendlier in legacy code

PR93253 – Document BOZ changes, make it friendlier in legacy code

        PR fortran/93253
        * check.c (gfc_invalid_boz): Mention -fallow-invalid-boz
        in the error message.
        * gfortran.texi (BOZ literal constants): List another missing
        extension and refer to -fallow-invalid-boz.
        * lang.opt (fallow-invalid-boz): Also mention 'X' in the help text
        as it is not covered by the previous wording.
        * primary.c (match_boz_constant): Tweak wording such that it is
        clear how to fix the nonstandard use.

        PR fortran/93253
        * fortran.dg/boz_7.f90: Updated dg-error.