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.

fuzz/Cargo.toml

@@ -21,18 +21,22 @@ libfuzzer-sys = { git = "https://github.com/rust-fuzz/libfuzzer-sys.git" }
 name = "compile"
 path = "fuzz_targets/compile.rs"
 test = false
+doc = false
 
 [[bin]]
 name = "instantiate"
 path = "fuzz_targets/instantiate.rs"
 test = false
+doc = false
 
 [[bin]]
 name = "instantiate_translated"
 path = "fuzz_targets/instantiate_translated.rs"
 test = false
+doc = false
 
 [[bin]]
 name = "api_calls"
 path = "fuzz_targets/api_calls.rs"
 test = false
+doc = false