Suggestion to add more details to DAST_EXCLUDE_RULES variable.

What does this MR do?

Add more details to the DAST_EXCLUDE_RULES variable description.

1. Adding a note about adding rule_id to DAST_EXCLUDE_RULES does not prevent the execution of the excluded rules but only suppress the alerts generated by the rule. Ref: #217419 (closed) (closed)
2. The link in the description for rule id reference is not the latest one. The current one is from the master branch of zap repo and was last updated on Oct '19. The latest one is in the dev branch, which is very frequently updated.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team