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

doc/coreutils.texi

@@ -2058,7 +2058,7 @@ Output at most @var{bytes} bytes of the input.  Prefixes and suffixes on
 @opindex --strings
 @cindex string constants, outputting
 Instead of the normal output, output only @dfn{string constants}: at
-least @var{bytes} consecutive ASCII graphic characters,
+least @var{bytes} consecutive printable characters,
 followed by a zero byte (ASCII NUL).
 Prefixes and suffixes on @var{bytes} are interpreted as for the
 @option{-j} option.

src/od.c

@@ -356,9 +356,8 @@ suffixes may be . for octal and b for multiply by 512.\n\
 "), stdout);
       fputs (_("\
   -N, --read-bytes=BYTES      limit dump to BYTES input bytes\n\
-  -S BYTES, --strings[=BYTES]  output strings of at least BYTES graphic chars;\
-\n\
-                                3 is implied when BYTES is not specified\n\
+  -S BYTES, --strings[=BYTES]  show only NUL terminated strings\n\
+                                of at least BYTES (3) printable characters\n\
   -t, --format=TYPE           select output format or formats\n\
   -v, --output-duplicates     do not use * to mark line suppression\n\
   -w[BYTES], --width[=BYTES]  output BYTES bytes per output line;\n\