Bug 1823813 - Add 'fuzzblockers' as examples of S2 bugs....

Bug 1823813 - Add 'fuzzblockers' as examples of S2 bugs. r=firefox-source-docs-reviewers,sylvestre DONTBUILD

Differential Revision: https://phabricator.services.mozilla.com/D173227

docs/bug-mgmt/guides/severity.rst

@@ -45,6 +45,8 @@ Bugs reported from QA
    are not working on several sites with ETP on (default). This is not an S1 since turning
    ETP off fixes the issue.
 
+Fuzzblocker bugs, which prevent fuzzing from making progress
+
 Examples of S3 bugs
 ^^^^^^^^^^^^^^^^^^^