rpc: remove 'exported or builtin' restriction for parameters (#20332)

* rpc: remove 'exported or builtin' restriction for parameters

There is no technial reason for this restriction because package reflect
can create values of any type. Requiring parameters and return values to
be exported causes a lot of noise in package exports.

* rpc: fix staticcheck warnings

rpc/doc.go

@@ -29,8 +29,6 @@ Methods that satisfy the following criteria are made available for remote access
 
  - method must be exported
  - method returns 0, 1 (response or error) or 2 (response and error) values
- - method argument(s) must be exported or builtin types
- - method returned value(s) must be exported or builtin types
 
 An example method:
 
@@ -74,13 +72,8 @@ An example server which uses the JSON codec:
  calculator := new(CalculatorService)
  server := NewServer()
  server.RegisterName("calculator", calculator)
-
  l, _ := net.ListenUnix("unix", &net.UnixAddr{Net: "unix", Name: "/tmp/calculator.sock"})
- for {
-	c, _ := l.AcceptUnix()
-	codec := v2.NewJSONCodec(c)
-	go server.ServeCodec(codec, 0)
- }
+ server.ServeListener(l)
 
 Subscriptions
 
@@ -90,7 +83,6 @@ criteria:
 
  - method must be exported
  - first method argument type must be context.Context
- - method argument(s) must be exported or builtin types
  - method must have return types (rpc.Subscription, error)
 
 An example method: