gRPC-Web name used to designate different things

(This isn't an issue with the grpc.io repo, but this seems like a convenient enough place to document the problem.)

It would seem that the gRPC-Web name used to designate different things. In particular:

1. https://github.com/grpc/grpc-web
2. https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-WEB.md, as mentioned in these pages:
- https://devblogs.microsoft.com/aspnet/grpc-web-for-net-now-available/
- https://docs.microsoft.com/en-us/aspnet/core/grpc/browser

Unless I'm mistaken, these are two different things, and if so, we should resolve this name reuse to avoid confusing users.

@thisisnotapril @stanley-cheung @Jennnnny