help: better explanations for some of the template filters

In particular, provide some example data for some of the common filters.
Thanks go to timeless again for pointing out where we were lacking.

mercurial/help.py

@@ -309,15 +309,20 b' PYTHONPATH::'
 
     - addbreaks: Any text. Add an XHTML "<br />" tag before the end of
           every line except the last.
-    - age: Date. Returns a human-readable age for the given date.
+    - age: Date. Returns a human-readable date/time difference between
+          the given date/time and the current date/time.
     - basename: Any text. Treats the text as a path, and returns the
-          basename. For example, "foo/bar/baz" becomes "baz".
-    - date: Date. Returns a date in a Unix date command format, including
+          last component of the path after splitting by the path
+          separator (ignoring trailing seprators). For example,
+          "foo/bar/baz" becomes "baz" and "foo/bar//" becomes "bar".
+    - date: Date. Returns a date in a Unix date format, including
           the timezone: "Mon Sep 04 15:13:13 2006 0700".
     - domain: Any text. Finds the first string that looks like an email
           address, and extracts just the domain component.
+          Example: 'User <[e-mail]>' becomes 'example.com'.
     - email: Any text. Extracts the first string that looks like an email
-          address.
+          address. Example: 'User <[e-mail]>' becomes
+          '[e-mail]'.
     - escape: Any text. Replaces the special XML/XHTML characters "&",
           "<" and ">" with XML entities.
     - fill68: Any text. Wraps the text to fit in 68 columns.
@@ -333,7 +338,7 b' PYTHONPATH::'
           in email headers.
     - short: Changeset hash. Returns the short form of a changeset hash,
           i.e. a 12-byte hexadecimal string.
-    - shortdate: Date. Returns a date like "2006-09-04".
+    - shortdate: Date. Returns a date like "2006-09-18".
     - strip: Any text. Strips all leading and trailing whitespace.
     - tabindent: Any text. Returns the text, with every line except the
           first starting with a tab character.