Formalize event topic extensions behavior in SEP-41 documentation

[leighmcculloch]

Hi all, I think we should formalize event topic extensions in SEP-41.

The Stellar Asset Contract emits SEP-41 events with an additional topic, the sep0011_asset field. It has been assumed by a few of us for sometime that any contract could extend the topics of a standard, but it's become apparent through conversations that this assumption is not obvious.

Even if this was, is, or could become an implicit understanding with any SEP contract's events, SEP-41 is so critical as a standard I think in a similar thinking as #1919 we should be explicit and remove this assumption from SEP-41.

I suggest adding a short paragraph to capture it:

Implementers may emit additional topics beyond those defined for each event.
To improve interoperability, it is recommended to do so as part of a SEP that
documents the behavior and when contracts should emit those additional topics.

PR: #1994