Need to update Code Metadata spec for function annotations #18

When we write the formal spec for this proposal, we will need to remember to update the Code Metadata specification (https://webassembly.github.io/spec/metadata/code/) to mention function annotations. Currently it only discusses instruction annotations. When we update the spec, we should make sure it follows the format used in the tool-conventions repo: https://github.com/WebAssembly/tool-conventions/blob/main/CodeMetadata.md. It may also be useful to add fuller examples to the Overview in this repo showing where the function annotations should be in relation to the functions.

Related tool conventions PR: WebAssembly/tool-conventions#263