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/misc/py/Cargo.toml

@@ -14,6 +14,7 @@ edition = "2018"
 name = "_wasmtime"
 crate-type = ["cdylib"]
 test = false
+doc = false
 
 [dependencies]
 wasmtime = { path = "../../api" }