wasm: add Note [Variable passing in JSFFI] as !13583 follow up

This patch adds a note to explain how the magic variables like
`__ghc_wasm_jsffi_dyld` are brought into scope of JSFFI code snippets,
as follow up work of !13583.

(cherry picked from commit f3bfe31e)
(cherry picked from commit 6f411dc5)