doc: clarify --help output for some optional arguments

* src/pr.c (usage): Fix the -S description to indicate
that the argument is optional for the short option.
* doc/coreutils.texi (pr invocation): Likewise.
* src/od.c (usage): Fix the -S description to indicate
that the argument is required for the short option.
Clarify -w takes an argument and that it is optional.

doc/coreutils.texi

@@ -2518,7 +2518,7 @@ three column options (@option{-COLUMN}|@option{-a -COLUMN}|@option{-m}) unless
 @option{-w} is set.  This is a @acronym{POSIX}-compliant formulation.
 
 
-@item -S@var{string}
+@item -S[@var{string}]
 @itemx --sep-string[=@var{string}]
 @opindex -S
 @opindex --sep-string
@@ -2528,8 +2528,8 @@ does not affect line truncation or column alignment.
 Without @option{-S}, and with @option{-J}, @command{pr} uses the default output
 separator, TAB@.
 Without @option{-S} or @option{-J}, @command{pr} uses a @samp{space}
-(same as @option{-S"@w{ }"}).  @option{--sep-string} with no
-@samp{=@var{string}} is equivalent to @option{--sep-string=""}.
+(same as @option{-S"@w{ }"}).
+If no @samp{@var{string}} argument is specified, @samp{""} is assumed.
 
 @item -t
 @itemx --omit-header

src/od.c

@@ -313,10 +313,10 @@ All arguments to long options are mandatory for short options.\n\
 "), stdout);
       fputs (_("\
   -N, --read-bytes=BYTES      limit dump to BYTES input bytes\n\
-  -S, --strings[=BYTES]       output strings of at least BYTES graphic chars\n\
+  -S BYTES, --strings[=BYTES]  output strings of at least BYTES graphic chars\n\
   -t, --format=TYPE           select output format or formats\n\
   -v, --output-duplicates     do not use * to mark line suppression\n\
-  -w, --width[=BYTES]         output BYTES bytes per output line\n\
+  -w[BYTES], --width[=BYTES]  output BYTES bytes per output line\n\
       --traditional           accept arguments in traditional form\n\
 "), stdout);
       fputs (HELP_OPTION_DESCRIPTION, stdout);

src/pr.c

@@ -250,7 +250,7 @@
                 compliant formulation. The source code translates -s into
                 the new options -S and -J, also -W if required.
 
-   -S STRING, --sep-string[=STRING]
+   -S[STRING], --sep-string[=STRING]
                 Separate columns by any string STRING. The -S option
                 doesn't react upon the -W/-w option (unlike -s option
                 does). It defines a separator nothing else.
@@ -2842,7 +2842,7 @@ Mandatory arguments to long options are mandatory for short options too.\n\
                     options (-COLUMN|-a -COLUMN|-m) except -w is set\n\
 "), stdout);
       fputs (_("\
-  -SSTRING, --sep-string[=STRING]\n\
+  -S[STRING], --sep-string[=STRING]\n\
                     separate columns by STRING,\n\
                     without -S: Default separator <TAB> with -J and <space>\n\
                     otherwise (same as -S\" \"), no effect on column options\n\