Vimdoc should document mappings

There should be a way to document individual mappings so they're listed in the mappings section of help files.

We should probably just have an explicit `@mapping KEYS [mode]` rather than trying to be clever and parse `:map` commands, at least for a first pass, because there are a lot of variants and a lot of code will use `:execute` and other high-level code structures to define mappings that might be hard to get right.