Tweak some API doc generation (#772)

* Build docs with the nightly toolchain so [foo::bar] links work by
  default. This is a relatively new feature of rustdoc and I thought it
  was stabilized at this point but apparently it's not!

* Tweak some API docs on `wasmtime::Strategy`

* Use `--all` to build all local crate documentation instead of trying
  to list the number of local crates

* Tweak some documentation configuration to avoid warnings generated by
  Cargo.

crates/wasi-c/Cargo.toml

@@ -32,3 +32,4 @@ maintenance = { status = "actively-developed" }
 [lib]
 test = false
 doctest = false
+doc = false