help: explain that revsets can be used where 1 or 2 revs are wanted

We did not seem to document that one can do things like "hg up :@"
where the last revision of the revset ":@".

mercurial/help/revisions.txt

@@ -29,6 +29,12 working directory is checked out, it is
 uncommitted merge is in progress, "." is the revision of the first
 parent.
 
+Finally, commands that expect a single revision (like ``hg update``) also
+accept revsets (see below for details). When given a revset, they use the
+last revision of the revset. A few commands accept two single revisions
+(like ``hg diff``). When given a revset, they use the first and the last
+revisions of the revset.
+
 Specifying multiple revisions
 =============================
 
@@ -188,3 +194,13 Some sample queries:
   release::
 
     hg log -r "(keyword(bug) or keyword(issue)) and not ancestors(tag())"
+
+- Update to commit that bookmark @ is pointing too, without activating the
+  bookmark (this works because the last revision of the revset is used)::
+
+    hg update :@
+
+- Show diff between tags 1.3 and 1.5 (this works because the first and the
+  last revisions of the revset are used)::
+
+    hg diff -r 1.3::1.5

tests/test-extension.t

@@ -731,14 +731,14 Test help topic with same name as extens
   $ echo "multirevs = multirevs.py" >> $HGRCPATH
 
   $ hg help multirevs | tail
+        bookmark (this works because the last revision of the revset is used):
   
-      - Changesets committed in May 2008, sorted by user:
+          hg update :@
   
-          hg log -r "sort(date('May 2008'), user)"
+      - Show diff between tags 1.3 and 1.5 (this works because the first and the
+        last revisions of the revset are used):
   
-      - Changesets mentioning "bug" or "issue" that are not in a tagged release:
-  
-          hg log -r "(keyword(bug) or keyword(issue)) and not ancestors(tag())"
+          hg diff -r 1.3::1.5
   
   use 'hg help -c multirevs' to see help for the multirevs command