target.h: Expands complete_target_initialization and add_target comments.

Expand a bit the comments to answer some questions I had when looking
at why a target of mine would not have some default methods set.

gdb/ChangeLog:

        * target.h (complete_target_initialization, add_target):
        Add comment.
read_cutu_die_from_dwo: Misc minor cleanups.

	* dwarf2read.c (read_cutu_die_from_dwo): Fix function comment.
	Remove unused local comp_dir_attr.  Assert exactly one of
	stub_comp_unit_die, stub_comp_dir is non-NULL.