revsets: add descriptions for sample queries in the help

mercurial/help/revsets.txt

@@ -168,10 +168,26 b' Command line equivalents for :hg:`log`::'
   -P x  ->  !::x
   -l x  ->  limit(expr, x)
 
-Some sample queries::
+Some sample queries:
+
+- Changesets on the default branch::
 
   hg log -r 'branch(default)'
+
+- Changesets on the default branch since tag 1.5 (excluding merges)::
+
   hg log -r 'branch(default) and 1.5:: and not merge()'
+
+- Changesets between tags 1.3 and 1.5 mentioning "bug" that affect
+  hgext/*::
+
   hg log -r '1.3::1.5 and keyword(bug) and file("hgext/*")'
+
+- Changesets in committed May 2008, sorted by user::
+
   hg log -r 'sort(date("May 2008"), user)'
+
+- Changesets mentioning "bug" or "issue" that are not in a tagged
+  release::
+
   hg log -r '(keyword(bug) or keyword(issue)) and not ancestors(tagged())'