Doc updates for --check

[carsonRadtke]

Fixes #5364 Update documentation regarding the output of the diff. Describe the difference between lines starting with +, -, and . Also added a snipped to the text printed by Rustfmt in the check optflag section.

Fixes #5376 Also documented that Rustfmt will exit with code 0 when reading from stdin despite seeing formatting errors. I found evidence of this being a design decision here: https://github.com/rust-lang/rustfmt/pull/2495. I guess the emacs package for Rust passes through stdin when formatting, but requires a 0 exit code. Although I should note that this doesn't appear to be the case today. I observed emacs not caring when Rustfmt returned 1 due to formatting issues.