Modify which symbols are documented (#42)

Motivation:

The .spi.yml was configured to exclude extended type to avoid
documenting the types from the re-exported `GRPCCore` module and the
core http2 module.

Modifications:

- Remove the config

Result:

Better docs

.spi.yml

@@ -3,5 +3,3 @@ builder:
   configs:
     - documentation_targets: [GRPCNIOTransportHTTP2, GRPCNIOTransportHTTP2Posix, GRPCNIOTransportHTTP2TransportServices]
       swift_version: 6.0
-      # Don't include @_exported types from GRPCCore
-      custom_documentation_parameters: [--exclude-extended-types]