cmd/gomobile: update gomobile bind -help text

The default ObjC prefix and Java package have been the empty string
for a while, but the gomobile bind -help text wasn't updated.

Change-Id: I8acb265ae7385121fae4c2cc314220d97575d20c
Reviewed-on: https://go-review.googlesource.com/99317
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

cmd/gomobile/bind.go

@@ -53,8 +53,8 @@ the module import wizard (File > New > New Module > Import .JAR or
 (File > Project Structure > Dependencies).  This requires 'javac'
 (version 1.7+) and Android SDK (API level 15 or newer) to build the
 library for Android. The environment variable ANDROID_HOME must be set
-to the path to Android SDK. The generated Java class is in the java
-package 'go.<package_name>' unless -javapkg flag is specified.
+to the path to Android SDK. Use the -javapkg flag to specify the Java
+package prefix for the generated classes.
 
 By default, -target=android builds shared libraries for all supported
 instruction sets (arm, arm64, 386, amd64). A subset of instruction sets
@@ -62,8 +62,8 @@ can be selected by specifying target type with the architecture name. E.g.,
 -target=android/arm,android/386.
 
 For -target ios, gomobile must be run on an OS X machine with Xcode
-installed. Support is not complete. The generated Objective-C types
-are prefixed with 'Go' unless the -prefix flag is provided.
+installed. The generated Objective-C types can be prefixed with the -prefix
+flag.
 
 For -target android, the -bootclasspath and -classpath flags are used to
 control the bootstrap classpath and the classpath for Go wrappers to Java