Document missing DAST Browserker documentation

Problem

Important features of the DAST browser-based analyzer are not documented.

Proposal

Document the following items, which were left over from the issue Rewrite the DAST browser-based documentation (#378990 - closed).

• Document request headers, and how they are only sent to target host/allowed hosts
• Document what "scope" means, and when it applies. It doesn't apply when authenticating
• Document mutual TLS
• Note somewhere that DAST does not validate SSL certificates

Anything else is out of scope and should be captured in an alternative issue.