Vimdoc should document mappings

[dbarnett]

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.