commandline-documentation: Refactor, purely

- remove makeCommand smart constructor
  (commands are easier to read/handle that way)
commandline-documentation: Rewrite/Add texts for many commands
commandline-documentation: Add and Modify two paragraphs
commandline-documentation: Add some relevant-config-keys
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: More description to user-config command
commandline-documentation: Fix comments in cabal-install/Main.hs
commandline-documentation: Add structure to command listing in `cabal help`