git-rev-list.txt: Clarify the use of multiple revision arguments

If one thinks of a revision as the set of commits which can be reached
from the rev, and of ^rev as the complement, then multiple arguments to
git rev-list can be neither understood as the intersection nor the union
of the individual sets.

But set language is the natural as well as logical language in which to
phrase this. So, add a paragraph which explains multiple arguments using
set language.

Suggested-by: Michael J Gruber <git@drmicha.warpmail.net>

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rev-list.txt

@@ -51,20 +51,26 @@ SYNOPSIS
 DESCRIPTION
 -----------
 
-Lists commit objects in reverse chronological order starting at the
-given commit(s), taking ancestry relationship into account.  This is
-useful to produce human-readable log output.
+List commits that are reachable by following the `parent` links from the
+given commit(s), but exclude commits that are reachable from the one(s)
+given with a '{caret}' in front of them.  The output is given in reverse
+chronological order by default.
 
-Commits which are stated with a preceding '{caret}' cause listing to
-stop at that point. Their parents are implied. Thus the following
-command:
+You can think of this as a set operation.  Commits given on the command
+line form a set of commits that are reachable from any of them, and then
+commits reachable from any of the ones given with '{caret}' in front are
+subtracted from that set.  The remaining commits are what comes out in the
+command's output.  Various other options and paths parameters can be used
+to further limit the result.
+
+Thus, the following command:
 
 -----------------------------------------------------------------------
 	$ git rev-list foo bar ^baz
 -----------------------------------------------------------------------
 
-means "list all the commits which are included in 'foo' and 'bar', but
-not in 'baz'".
+means "list all the commits which are reachable from 'foo' or 'bar', but
+not from 'baz'".
 
 A special notation "'<commit1>'..'<commit2>'" can be used as a
 short-hand for "{caret}'<commit1>' '<commit2>'". For example, either of