improve the --file-lines help

[richardmon]

This is something incredibly small but as someone new to using rustfmt I found the --file-lines example quite frustrating

rustfmt --file-lines '[
    {"file":"src/lib.rs","range":[7,13]},
    {"file":"src/lib.rs","range":[21,29]},
    {"file":"src/foo.rs","range":[10,11]},
    {"file":"src/foo.rs","range":[15,15]}]' 

This leads me to believe that the file is going to be taken based on the json I passed as an argument but that is not the case, it also need to the files that are going to be modified as extra parameters, so since this --help=file-lines represents a complete example the message should be something like this:

rustfmt --file-lines '[
    {"file":"src/lib.rs","range":[7,13]},
    {"file":"src/lib.rs","range":[21,29]},
    {"file":"src/foo.rs","range":[10,11]},
    {"file":"src/foo.rs","range":[15,15]}]' src/lib.rs src/foo.rs

[ytmimi]

@richardmon thanks for your first contribution to rustfmt 🎉

When you have a chance, can you rewrite the example like:

rustfmt src/lib.rs src/foo.rs --file-lines '[
    {"file":"src/lib.rs","range":[7,13]},
    {"file":"src/lib.rs","range":[21,29]},
    {"file":"src/foo.rs","range":[10,11]},
    {"file":"src/foo.rs","range":[15,15]}]'

I think it's a little cleaner if the file names come first. Besides that I'm happy to make this change. You also need to add the --unstable-features flag in order to use --file-lines, right?

[richardmon]

Done!, thanks.

[calebcartwright]

IIRC it may not be necessary to explicitly provide src/foo.rs as an input file, provided the example src/lib.rs imports foo, but regardless I agree that while minor this is a helpful change