Review and update content about SameSite cookies

[rbsec]

There are few areas of the guide that don't really take into account the current state of SameSite cookies:

• The Testing Cookie Attributes guide says that "most" browsers" default to Lax, but this isn't true for Firefox or Safari.
• The Cross-Site Request Forgery guide doesn't mention it at all, and the PoCs given won't work in some browsers because of it.
• The Cross-Site Script Inclusion guide doesn't mention it at all, and the PoC given won't work in some (all? #954) browsers because of it.

There might also be some other areas that I've missed?

[mademarc]

Hey @rbsec Can in contribute and help out on this issue that needs revise?

Please let know when possible

[kingthorin]

Absolutely. Just let us know if you have questions or if there's enough detail there to get you started.

[mademarc]

Thanks @kingthorin and i just message you some questions via slack, did you received them?