Users coming from other languages (namely C and C++) often expect
to use a -Wall flag. Rustc doesn't support that, and previously it
simply printed that it didn't recognize the "all" lint.

This change makes rustc print out a help message, explaining:
- Why there is no -Wall flag
- How to view all the available warnings
- Point out that the most commonly used warning is -Wunused
- Instead of using a command-line flag, the user should consider
  a !#[warn(unused)] directive in the root of their crate.

The reference to -Wunused was removed, and some phrasing was changed.