help: add web commands to help documentation The capabilities and URL endpoints of the hgweb server can currently only be inferred by looking at links in `hg serve` output or by reading the source code. I've frequently found myself wanting to quickly see what URLs and capabilities are available. This patch teaches the help system how to display information about web commands and their URLs. Using a mechanism similar to revsets, templates, etc, we can now iterate over the docstrings of registered web command functions and display them in the help output. Unfortunately, web commands don't currently have docstrings, so the output is currently empty. This will be addressed in the following patches. I apologize for the patch bomb.