commandline-documentation: Fix comments in cabal-install/Main.hs
commandline-documentation: Change UserHooks description
commandline-documentation: More description to user-config command
Change globalCommands description generation

- previously, runCommands internally modified the description
  by adding the list of commands.
- now, globalCommands is a function that builds the complete
  description without need for changes during runCommands invocation.
- slightly more code duplication, but better separation of concerns
commandline-documentation: Add structure to command listing in `cabal help`