Mention the `warnings` lint group in `rustc -W help` text

[Manishearth]

rustc -W help mentions:

Lint groups provided by rustc:

         name  sub-lints
         ----  ---------
    bad-style  non-camel-case-types, non-snake-case, non-upper-case-globals
       unused  unused-imports, unused-variables, unused-assignments, dead-code, unused-mut, unreachable-code, unused-must-use, unused-unsafe, path-statements, unused-attributes

There's another lint group, warnings, which turns off all builtin lints. We should mention it in this list (preferably at the top), or in another line below.

Relevant code:

rust/src/librustc_driver/lib.rs

Line 670 in ac0e845

let print_lint_groups = |lints: Vec<(&'static str, Vec<lint::LintId>)>| {

If you wish to make it appear in the table, ensure that max_name_len takes the presence of warnings into account too. Instead of listing the lints for the second column, add some text along the lines of "all builtin lints".

[faineance]

Can I take this?

[Manishearth]

Sure! Let me know if you need help!

(assigning to self since I can't assign it to you)

[athaeryn]

@faineance are you working on this? If you're not, I'd like to take it!

[huonw]

Hm, @Manishearth, where is this lint group defined?

[Manishearth]

It's not. It's magically in built, and the help text is similarly hand-crafted.