doc: od --strings: clarify operation

doc: od --strings: clarify operation

* doc/coreutils.texi (od invocation): Remove mention of ASCII,
as all printable characters in unibyte locales are output.
* src/od.c (usage): Clarify that only NUL terminated strings
are displayed, and that it's printable chars, not only graphic chars
that are output. I.e., spaces are output also if part of the string.
Reported at https://bugs.ddebian.org/1037217