[DOC] Update the sanitizer documentation

Summary:
Passing the suppressions manually is no longer needed when using the
check-* targets (since D5689). Update the documentation to reflect
this.

Test Plan: Read the doc and try the examples.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5760