Unify rawPayload message docs

[svix-jplatte]

Motivation

In previous PRs for https://github.com/svix/monorepo-private/issues/9470, some review comments were made for better docs. I also just failed and used different ways of saying the same thing in different places. This all made for pointlessly diverging docs between the different languages.

Solution

Use the same basic formulation everywhere: "pre-serialized payload", same description of the "as-is" nature of raw payloads, same description of the content-type parameter. A number of language-specific docs remain to cover differences in how these functions work (e.g. for optional content-type, Java has two overloads of one function, Rust has a builder-style method, most other languages use an optional parameter).