[spec] externaddr is meant to be uninterpreted but has a grammar definition

[anuraaga]

I was looking through the definition of addresses and found it might be contradicting itself.

https://www.w3.org/TR/wasm-core-2/exec/runtime.html#syntax-externaddr

It mentions "an embedder may supply an uninterpreted set of host addresses" but also has "externaddr ::= addr", which seems to be interpreting the host address as an unsigned integer. Two possible improvements are

• Rename "host addresses" to "external addresses", or "externaddr" to "hostaddr". I think these two are talking about the same thing but it's not clear with the word being different.
• Tweak "externaddr ::= addr", I don't know if there is a way to represent an opaque value in the grammar but "addr" doesn't seem correct here given it has a definition