help: document bundle specifications I softly formalized the concept of a "bundle specification" a while ago when I was working on clone bundles and stream clone bundles and wanted a more robust way to define what exactly is in a bundle file. The concept has existed for a while. Since it is part of the clone bundles feature and exposed to the user via the "-t" argument to `hg bundle`, it is something we need to support for the long haul. After the 4.1 release, I heard a few people comment that they didn't realize you could generate zstd bundles with `hg bundle`. I'm partially to blame for not documenting it in bundle's docstring. Additionally, I added a hacky, experimental feature for controlling the compression level of bundles in 76104a4899ad. As the commit message says, I went with a quick and dirty solution out of time constraints. Furthermore, I wanted to eventually store this configuration in the "bundlespec" so it could be made more flexible. Given: a) bundlespecs are here to stay b) we don't have great documentation over what they are, despite being a user-facing feature c) the list of available compression engines and their behavior isn't exposed d) we need an extensible place to modify behavior of compression engines I want to move forward with formalizing bundlespecs as a user-facing feature. This commit does that by introducing a "bundlespec" help page. Leaning on the just-added compression engine documentation and API, the topic also conveniently lists available compression engines and details about them. This makes features like zstd bundle compression more discoverable. e.g. you can now `hg help -k zstd` and it lists the "bundlespec" topic.