Please document http signature changes in security pages

[wmurphyrd]

updated after tracking down issue
Hello, I've received an issue report that Masoton 3.2.1 servers are refusing my POSTs from gup.pe with message Mastodon requires the Digest header to be signed when doing a POST request

Current documentation at https://docs.joinmastodon.org/spec/security/#http appears to be out of date

Could you update the page so I know exactly what is required now?

[ClearlyClaire]

The documentation change in 34fab9a is welcome, but it doesn't solve the issue at hand.
https://docs.joinmastodon.org/spec/security/#http-sign should be updated to say that Mastodon requires the following (pseudo-)headers:

• Date or (created)
• Host
• (request-target)
• Digest if it's a POST request