More precise documentation about underscores and mnemonics. (#66380)

More precise documentation about underscores and mnemonics. (#66380)

        * gtk/gtklabel.c (gtk_label_new_with_mnemonic),
        gtk/gtkbutton.c (gtk_button_new_with_mnemonic): More precise
        documentation about underscores and mnemonics. (#66380)

        * gtk/gtktextiter.c (gtk_text_iter_backward_word_starts): Fix
        cyclic reference in docs.

        * gtk/gtklabel.c (gtk_label_set_justify): Correct documentation
        of default value. (#65402)

        * gtk/gtkmain.c (gtk_set_locale, gtk_disable_set_locale):
        Markup fixes.

        * gdk-pixbuf-io.c, gdk-pixbuf-animation.c, gdk-pixbuf-data.c,
        gdk-pixbuf-loader.c, gdk-pixbuf-scale.c, gdk-pixbuf-util.c,
        gdk-pixdata.c: Markup fixes.

        * gtk/text_widget.sgml: More precise wording. (#63388)

        * gtk/tmpl/gtksignal.sgml (GTK_SIGNAL_OFFSET): Add docs.

        * gtk/resources.sgml: Fix markup of mail URLs.

        * gtk/tmpl/gtkpaned.sgml, gtk/tmpl/gtkobject.sgml: Markup fixes.

        * gtk/tmpl/gtktoolbar.sgml (gtk_toolbar_{prepend,append}_element):
        Expand documentation. (#60471)

        * gtk/tmpl/gtkmain.sgml: Remove misleading information about
        gtk_set_locale().  (#65758)