x/mobile: copy comments to generated headers

[joseelin]

use golang build mobile library for other developers,mybe include comments better
source code

// NSDictionary for en-us
type NSDictionary interface {
	// Get get value
	Get(key string) string
	// Has has value
	Has(key string) bool
}

command:gomobile bind -v -target=ios ...
output framework header

@interface HttpNSDictionary : NSObject <goSeqRefInterface, HttpNSDictionary> {
}
@property(strong, readonly) id _ref;

- (instancetype)initWithRef:(id)ref;
- (NSString*)get:(NSString*)key;
- (BOOL)has:(NSString*)key;
@end

want include comments

//for en-us
@interface HttpNSDictionary : NSObject <goSeqRefInterface, HttpNSDictionary> {
}
@property(strong, readonly) id _ref;

- (instancetype)initWithRef:(id)ref;
//get value
- (NSString*)get:(NSString*)key;
//has value
- (BOOL)has:(NSString*)key;
@end

[bcmills]

CC: @eliasnaur

[gopherbot]

Change https://golang.org/cl/114056 mentions this issue: cmd/gobind: copy documentation to generated source

[gopherbot]

Change https://golang.org/cl/114055 mentions this issue: bind: copy documentation for interface methods to ObjC