cmd/compile: improve compiler directive docs
This section has gotten long enough that it deserves to be multiple
sections. This also allows us to better structure information shared
by subsets of directives. In particular, this enables a self-contained
section on the wasm directives.
Updates #66984.
Change-Id: I062081d46c6b0aef7887fdaf9efae80f32ad4b21
Reviewed-on: https://go-review.googlesource.com/c/go/+/638881
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Austin Clements <austin@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>