The WASM UDF implementation has changed since the last time the docs
were written. In particular, the Rust helper library has been
released, and using it should be the recommended method.
Some decisions that were only experimental at the start, were also
"set in stone", so we should refer to them as such.
The docs also contain some code examples. This patch adds tests for
these examples to make sure that they are not wrong and misleading.
Closes#12941
