Tiny changes.

* src/misc.c: Fix comments, rename wd.cwd to wd.abspath (Nathan Stratton
Treadway);
* src/tar.c (options): Reword description of the --starting-file and
--preserve-order options.
(decode_options): Both --starting-file and --preserve-order have meaning
only when used together with an archive reading command. (Pavel Raiskup).