Improve doc for the default undo manager

Explain the default undo manager in the description of GtkSourceBuffer.
The explanations in the functions were not correct if a custom undo
manager is used.
Fix GtkSourceMark documentation

gtk_source_view_set_mark_category_pixbuf() and
gtk_source_view_set_mark_category_priority() don't exist. Replace them
by gtk_source_view_set_mark_attributes().
Document GtkSourceGutterRendererAlignmentMode

Avoid a warning during the doc generation.