revisions.txt: describe 'rev1 rev2 ...' meaning for ranges

The "Specifying ranges" section does not mention explicitly that
several commits can be specified to form a range.

Add a mention to that effect.

Signed-off-by: Philippe Blain <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: phil-blain

Documentation/revisions.txt

@@ -254,6 +254,9 @@ specifying a single revision, using the notation described in the
 previous section, means the set of commits `reachable` from the given
 commit.
 
+Specifying several revisions means the set of commits reachable from
+any of the given commits.
+
 A commit's reachable set is the commit itself and the commits in
 its ancestry chain.