Random Doc Improvements

[ethanfrey]

Various items to improve text-wise

• [x] "known vector attacks " -> "known attack vectors"
• [x] when mentioning decimal and blake2b in the docs, it might make sense to mention why each is useful. By the way, blake3 is out now, which has significant parts written in rust
• [x] early in the docs, on the Actor model page, the code is in Go. If this is for people writing Rust, it might be distracting or confusing. https://www.cosmwasm.com/docs/intro/actor
• [x] on the canonical addresses page, the ?= operator may not be clear for some readers https://www.cosmwasm.com/docs/intro/addresses#canonical-addresses