Do not needlessly promote allocUnsafe.

Dates back to #4682

While the intention was to make allocUnsafe a code replacement for Buffer constructor, it has also been to let users use Buffer.alloc by default and/or when they are not sure, and optimize with allocUnsafe only when needed and when they ensured that nothing gets leaked (which often requires extra actions e.g. error handling or guards against race conditions)

Most documentation methods should use Buffer.alloc instead of Buffer.allocUnsafe by default in examples

This aligns documentation accents internally with the wording in this intent:

And https://nodejs.org/docs/latest/api/buffer.html#bufferfrom-bufferalloc-and-bufferallocunsafe