[linter] Update writing lints doc for current process

Contributes to #59696 by updating the writing lint docs for the current process.
I left updating the bootstrapping tool for a separate CL to keep this easier to review.

Based on what direction we go for the messages.yaml file, this doc will continue to change and hopefully get simpler.

Change-Id: I706e9438fbd6acdba41dd7198ad3b5afd5acbe4e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/400420
Commit-Queue: Phil Quitslund <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>
Reviewed-by: Phil Quitslund <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: parlough