Doc: small fixes in GtkTextView

Move GtkWrapMode from GtkTextTag to the GtkTextView section. The wrap
mode property is in the text view.

Links to the "mark-set" and "mark-deleted" signals.

Add a precision about gtk_text_buffer_get_iter_at_line().

Fix typo in gtk_text_tag_set_priority().

https://bugzilla.gnome.org/show_bug.cgi?id=708076