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.

.github/workflows/main.yml

@@ -46,17 +46,9 @@ jobs:
       with:
         submodules: true
     - uses: ./.github/actions/install-rust
-    - run: cargo doc --no-deps -p wasmtime-cli
-    - run: cargo doc --no-deps -p wasmtime
-    - run: cargo doc --no-deps -p wasmtime-debug
-    - run: cargo doc --no-deps -p wasmtime-environ
-    - run: cargo doc --no-deps -p wasmtime-interface-types
-    - run: cargo doc --no-deps -p wasmtime-jit
-    - run: cargo doc --no-deps -p wasmtime-obj
-    - run: cargo doc --no-deps -p wasmtime-runtime
-    - run: cargo doc --no-deps -p wasmtime-wasi
-    - run: cargo doc --no-deps -p wasmtime-wast
-    - run: cargo doc --no-deps -p wasi-common
+      with:
+        toolchain: nightly
+    - run: cargo doc --no-deps --all --exclude wasmtime-cli --exclude test-programs
     - uses: actions/upload-artifact@v1
       with:
         name: doc-api