Clarify if forgiving-base64 is allowed

Derivative of https://github.com/ruma/ruma/issues/591;

`Construct` sent a base64 string in `signatures` that synapse accepted, it can still be accepted by using [forgiving base64](https://infra.spec.whatwg.org/#forgiving-base64-decode), arguably "non-compliant", yet accepted by https://github.com/matrix-org/python-unpaddedbase64.

Rust crate `base64` snagged on it, and gave a way out with a mention to above link about "forgiving base64", is this allowed per the matrix spec? If so, I think a comment should be made about that.

Read the linked Matrix issue #591 and the Matrix specification around handling base64 strings in `signatures`, then compare the relevant requirement with the forgiving base64 behavior linked in the issue. Determine whether the spec permits this behavior; if it does, document that clarification in the relevant specification text. Done means the spec clearly resolves whether forgiving base64 is allowed.

Written by the indexing model from the issue text.