Documentation: Update to show new config options

  remote-execution:

    # A url defining a remote execution server

    execution-service:

      url: http://buildserver.example.com:50051

    storage-service:

    - url: https://foo.com:11002/

      server-cert: server.crt

      client-cert: client.crt

      client-key: client.key

The execution-service part of remote execution does not support encrypted

connections yet, so the protocol must always be http.

storage-service specifies a remote CAS store and the parameters are the

same as those used to specify an :ref:`artifact server <artifacts>`.

The url should contain a hostname and port separated by ':'. Only plain HTTP is

currently suported (no HTTPS).

The storage service may be the same endpoint used for artifact

caching. Remote execution cannot work without push access to the

storage endpoint, so you must specify a client certificate and key,

and a server certificate.

The Remote Execution API can be found via https://github.com/bazelbuild/remote-apis.