[patch, committed] Update documentation

[Jerry D]

Committed.

commit a38310ca138319c463b6892cc723970a37aeee9e (HEAD -> master, origin/master, 
origin/HEAD)
Author: Jerry DeLisle <jvdeli...@gcc.gnu.org>
Date:   Sat Jan 17 09:23:53 2026 -0800
    Fortran: Documentation update on -fdefault-real-8 and family

            PR fortran/122957

    gcc/fortran/ChangeLog:

            * invoke.texi: Add note that these features are incompatible
            with user defined derived type I/O. (DTIO)