Various fixes in the documentation
* doc/tar.1: Add missing dots, use plural when necessary,
tweak a wording. Remove an incorrect observation, three times.
Add some missing articles, correct some formatting,
and expand the opaque descriptions of two options.
* doc/tar.texi: Drop a stray `cd` command from an example.
Correct two cross references, correct the paragraph
about the manpage, and unbreak a URL.
* src/names.c: Correct and shorten an error message: "non-optional"
means "mandatory", but "non-option" is what was meant. And the
phrase "in archive create or update mode" was both unneeded and
incomplete.
* tests/positional01.at: Change expected error text.
* tests/positional02.at: Likewise.
* tests/positional03.at: Likewise.