grpc: Improve documentation of read/write buffer size server and dial options (#5800)

Fixes https://github.com/grpc/grpc-go/issues/5798

dialoptions.go

@@ -116,8 +116,9 @@ func newFuncDialOption(f func(*dialOptions)) *funcDialOption {
 // be twice the size to keep syscalls low. The default value for this buffer is
 // 32KB.
 //
-// Zero will disable the write buffer such that each write will be on underlying
+// Zero or negative values will disable the write buffer such that each write
-// connection. Note: A Send call may not directly translate to a write.
+// will be on underlying connection. Note: A Send call may not directly
+// translate to a write.
 func WithWriteBufferSize(s int) DialOption {
 	return newFuncDialOption(func(o *dialOptions) {
 		o.copts.WriteBufferSize = s
@@ -127,8 +128,9 @@ func WithWriteBufferSize(s int) DialOption {
 // WithReadBufferSize lets you set the size of read buffer, this determines how
 // much data can be read at most for each read syscall.
 //
-// The default value for this buffer is 32KB. Zero will disable read buffer for
+// The default value for this buffer is 32KB. Zero or negative values will
-// a connection so data framer can access the underlying conn directly.
+// disable read buffer for a connection so data framer can access the
+// underlying conn directly.
 func WithReadBufferSize(s int) DialOption {
 	return newFuncDialOption(func(o *dialOptions) {
 		o.copts.ReadBufferSize = s

server.go

@@ -233,10 +233,11 @@ func newJoinServerOption(opts ...ServerOption) ServerOption {
 	return &joinServerOption{opts: opts}
 }
 
-// WriteBufferSize determines how much data can be batched before doing a write on the wire.
+// WriteBufferSize determines how much data can be batched before doing a write
-// The corresponding memory allocation for this buffer will be twice the size to keep syscalls low.
+// on the wire. The corresponding memory allocation for this buffer will be
-// The default value for this buffer is 32KB.
+// twice the size to keep syscalls low. The default value for this buffer is
-// Zero will disable the write buffer such that each write will be on underlying connection.
+// 32KB. Zero or negative values will disable the write buffer such that each
+// write will be on underlying connection.
 // Note: A Send call may not directly translate to a write.
 func WriteBufferSize(s int) ServerOption {
 	return newFuncServerOption(func(o *serverOptions) {
@@ -244,11 +245,10 @@ func WriteBufferSize(s int) ServerOption {
 	})
 }
 
-// ReadBufferSize lets you set the size of read buffer, this determines how much data can be read at most
+// ReadBufferSize lets you set the size of read buffer, this determines how much
-// for one read syscall.
+// data can be read at most for one read syscall. The default value for this
-// The default value for this buffer is 32KB.
+// buffer is 32KB. Zero or negative values will disable read buffer for a
-// Zero will disable read buffer for a connection so data framer can access the underlying
+// connection so data framer can access the underlying conn directly.
-// conn directly.
 func ReadBufferSize(s int) ServerOption {
 	return newFuncServerOption(func(o *serverOptions) {
 		o.readBufferSize = s