Convert Go documentation to JavaDoc tags (/** ... */).
Since the .aar file format doesn't support source files, gomobile
will create a package-sources.jar along with the main package.aar.
For Objective-C, JavaDoc-style comments seems to work as well,
judging by manual inspection of Xcode quick help.
Change-Id: I47fe5b6804681d459a873be37a44610d392166ef
Reviewed-on: https://go-review.googlesource.com/52330
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>