Add details on function pointers to BasicCABI

I found BasicCABI when looking for documentation on what conventions LLVM uses when compiling to Wasm. I noticed that it doesn't seem to include details on function pointers.

In my own guessing by looking at the details of a compiled result, I noticed that a Wasm binary has a single table of type=funcref, elements in the table with indexes to functions, and when a function is called with a function pointer argument, the index of the element in the table is used. One point I'd like to confirm is my assumption that funcrefs always have to be in the first table in the binary as there is no table index involved when invoking.

Would it be appropriate to add information on this to BasicCABI? I can try a PR if my understanding seems correct but it may be better for someone who implemented that in LLVM to do so.

Start with BasicCABI.md and review the function-pointer behavior described in the issue against the relevant compiled Wasm output and LLVM conventions. Confirm the table and funcref assumptions, then document the confirmed function-pointer convention in BasicCABI.md; no specific test is mentioned.