doc: clarify the output format for the *sum utilities

* src/md5sum.c (usage): Detail the reasons for the default
double space between checksum and file name.
* doc/coreutils.texi (md5sum invocation): Likewise.
Explicitly mention the 3 formats that --check supports.

Fixes http://bugs.gnu.org/19725

doc/coreutils.texi

@@ -4057,8 +4057,11 @@ consistent.  Synopsis:
 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
 @end example
 
-For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
-indicating binary or text input mode, and the file name.
+For each @var{file}, @samp{md5sum} outputs by default, the MD5 checksum,
+a space, a flag indicating binary or text input mode, and the file name.
+Binary mode is indicated with @samp{*}, text mode with @samp{ } (space).
+Binary mode is the default on systems where it's significant,
+otherwise text mode is the default.
 If @var{file} contains a backslash or newline, the
 line is started with a backslash, and each problematic character in
 the file name is escaped with a backslash, making the output
@@ -4089,9 +4092,11 @@ Read file names and checksum information (not data) from each
 whether the checksums match the contents of the named files.
 The input to this mode of @command{md5sum} is usually the output of
 a prior, checksum-generating run of @samp{md5sum}.
-Each valid line of input consists of an MD5 checksum, a binary/text
-flag, and then a file name.
-Binary mode is indicated with @samp{*}, text with @samp{ } (space).
+Three input formats are supported.  Either the default output
+format described above, the @option{--tag} output format,
+or the BSD reversed mode format which is similar to the default mode,
+but doesn't use a character to distinguish binary and text modes.
+@sp 1
 For each such line, @command{md5sum} reads the named file and computes its
 MD5 checksum.  Then, if the computed message digest does not match the
 one on the line with the file name, the file is noted as having

src/md5sum.c

@@ -206,9 +206,9 @@ The following four options are useful only when verifying checksums:\n\
       printf (_("\
 \n\
 The sums are computed as described in %s.  When checking, the input\n\
-should be a former output of this program.  The default mode is to print\n\
-a line with checksum, a character indicating input mode ('*' for binary,\n\
-space for text), and name for each FILE.\n"),
+should be a former output of this program.  The default mode is to print a\n\
+line with checksum, a space, a character indicating input mode ('*' for binary,\
+\n' ' for text or where binary is insignificant), and name for each FILE.\n"),
               DIGEST_REFERENCE);
       emit_ancillary_info (PROGRAM_NAME);
     }