Apply suggestions from code review

Co-authored-by: Who? Me?! <mark-i-m@users.noreply.github.com>
Co-authored-by: Chris Simpkins <git.simpkins@gmail.com>

Author: 3 people

src/diagnostics.md

@@ -75,7 +75,7 @@ The descriptions are written in markdown, and all of them are linked in the
 
 <!-- TODO: When should an error use an error code, and when shouldn't it? -->
 
-[`librustc_error_codes`]: https://github.com/rust-lang/rust/blob/master/src/librustc_error_codes/error_codes.rs
+[`librustc_error_codes`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_error_codes/error_codes/index.html
 [error index]: https://doc.rust-lang.org/error-index.html
 [RFC 1567]: https://github.com/rust-lang/rfcs/blob/master/text/1567-long-error-codes-explanation-normalization.md
 
@@ -104,7 +104,7 @@ use an error-level lint instead of a fixed error.
   flag. That said, don't make it so terse that it's hard to understand.
 - The word "illegal" is illegal. Prefer "invalid" or a more specific word
   instead.
-- Errors should document the span of code where they occur – the `span_..`
+- Errors should document the span of code where they occur – the `librustc_errors::diagnostic_builder::DiagnosticBuilder`  `span_*` 
   methods allow to easily do this. Also `note` other spans that have
   contributed to the error if the span isn't too large.
 - When emitting a message with span, try to reduce the span to the smallest