help.hgweb: add a section describing URLs and common parameters

The behavior of hgweb's URLs isn't documented anywhere inside Mercurial.
Let's start to change that.

mercurial/help/hgweb.txt

@@ -1,50 +1,79 b''
 Mercurial's internal web server, hgweb, can serve either a single
 repository, or a tree of repositories. In the second case, repository
 paths and global options can be defined using a dedicated
 configuration file common to :hg:`serve`, ``hgweb.wsgi``,
 ``hgweb.cgi`` and ``hgweb.fcgi``.
 
 This file uses the same syntax as other Mercurial configuration files
 but recognizes only the following sections:
 
   - web
   - paths
   - collections
 
 The ``web`` options are thoroughly described in :hg:`help config`.
 
 The ``paths`` section maps URL paths to paths of repositories in the
 filesystem. hgweb will not expose the filesystem directly - only
 Mercurial repositories can be published and only according to the
 configuration.
 
 The left hand side is the path in the URL. Note that hgweb reserves
 subpaths like ``rev`` or ``file``, try using different names for
 nested repositories to avoid confusing effects.
 
 The right hand side is the path in the filesystem. If the specified
 path ends with ``*`` or ``**`` the filesystem will be searched
 recursively for repositories below that point.
 With ``*`` it will not recurse into the repositories it finds (except for
 ``.hg/patches``).
 With ``**`` it will also search inside repository working directories
 and possibly find subrepositories.
 
 In this example::
 
   [paths]
   /projects/a = /srv/tmprepos/a
   /projects/b = c:/repos/b
   / = /srv/repos/*
   /user/bob = /home/bob/repos/**
 
 - The first two entries make two repositories in different directories
   appear under the same directory in the web interface
 - The third entry will publish every Mercurial repository found in
   ``/srv/repos/``, for instance the repository ``/srv/repos/quux/``
   will appear as ``http://server/quux/``
 - The fourth entry will publish both ``http://server/user/bob/quux/``
   and ``http://server/user/bob/quux/testsubrepo/``
 
 The ``collections`` section is deprecated and has been superseded by
 ``paths``.
+
+URLs and Common Arguments
+=========================
+
+URLs under each repository have the form ``/{command}[/{arguments}]``
+where ``{command}`` represents the name of a command or handler and
+``{arguments}`` represents any number of additional URL parameters
+to that command.
+
+The web server has a default style associated with it. Styles map to
+a collection of named templates. Each template is used to render a
+specific piece of data, such as a changeset or diff.
+
+The style for the current request can be overwritten two ways. First,
+if ``{command}`` contains a hyphen (``-``), the text before the hyphen
+defines the style. For example, ``/atom-log`` will render the ``log``
+command handler with the ``atom`` style. The second way to set the
+style is with the ``style`` query string argument. For example,
+``/log?style=atom``. The hyphenated URL parameter is preferred.
+
+Not all templates are available for all styles. Attempting to use
+a style that doesn't have all templates defined may result in an error
+rendering the page.
+
+Many commands take a ``{revision}`` URL parameter. This defines the
+changeset to operate on. This is commonly specified as the short,
+12 digit hexidecimal abbreviation for the full 40 character unique
+revision identifier. However, any value described by
+:hg:`help revisions` typically works.