Add clarification to API behavior with respect to CSRF and cookies

[manedurphy]

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What does this PR aim to accomplish?:

• Adds clarity on how the Pi-hole API validates requests with Cookie authentication and CSRF tokens.
• The docs mentions that cookie-based authentication must include a CSRF token in the X-FTL-CSRF header, but the code shows that the header is named X-CSRF-TOKEN, see here.
• The code examples show the use of the X-FTL-CSRF in scenarios where the code/script run outside of the browser. This is misleading, as those example do not explicitly opt-in for cookie-based authentication.
• A JavaScript example showed setting the sid value in the request URI in additional to setting the CSRF token, which is also misleading because in a browser we would expect the cookie to be set automatically, which the API checks before checking the request URI.

How does this PR accomplish the above?:

• All changes address the following points mentioned above

Link documentation PRs if any are needed to support this PR:

• N/A

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[dschaper]

@DL6ER Can you check this over and see if it lines up with the API design please?