doc: clarify sort --key handling of default field separators

* doc/coreutils.texi (sort invocation): Mention in the summary
dicussion that --key is used to specify fields.  Give a summary
in the --key description, of the most common use case of specifying
a field, and that by default those fields include the blank separators
at the start of each field in the comparisons.

1 files changed, 9 insertions, 2 deletions

diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 6a671bb8e..47c63db79 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -4022,7 +4022,7 @@ sort [@var{option}]@dots{} [@var{file}]@dots{}
 Many options affect how @command{sort} compares lines; if the results
 are unexpected, try the @option{--debug} option to see what happened.
 A pair of lines is compared as follows:
-@command{sort} compares each pair of fields, in the
+@command{sort} compares each pair of fields (see @option{--key}), in the
 order specified on the command line, according to the associated
 ordering options, until a difference is found or no fields are left.
 If no key fields are specified, @command{sort} uses a default key of
@@ -4332,7 +4332,14 @@ Specify a sort field that consists of the part of the line between
 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
 omitted), @emph{inclusive}.
 
-Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
+In its simplest form @var{pos} specifies a field number (starting with 1),
+with fields being separated by runs of blank characters, and by default
+those blanks being included in the comparison at the start of each field.
+To adjust the handling of blank characters see the @option{-b} and
+@option{-t} options.
+
+More generally,
+each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
 where @var{f} is the number of the field to use, and @var{c} is the number
 of the first character from the beginning of the field.  Fields and character
 positions are numbered starting with 1; a character position of zero in