setcookie: Add note regarding SameSite=None and disabled Secure behavior

[AllenJB]

Fixes #4873

[TimWolla]

@Girgias might have markup opinions here?

[Girgias]

Indeed I have some markup notes, the wrapping para is very much unnecessary and please use <simpara> and fix the above note at the same time :)

[AllenJB]

In this case fixing this instance would have created an inconsistency with the markup across rest of this file, so I've reviewed the whole file to make it consistent.

[Girgias]

Sorry for the delay, but going throught the file just brings to light more issues :/

[AllenJB]

I've fixed all the review issues, including CDATA indents and trailing whitespace.

Additionally, I changed the suggestion to consider explode() for setting multiple values in a single cookie to json_encode() as I think this is easier / less error prone to use correctly.