Update @IntDef doc to suggest SOURCE retention.

CLASS retention is unnecessary because IDE analysis cannot rely on
looking at the annotation in the .class files, and instead extracts
relevant information from the source code itself.

Note that the documentation for @StringDef already suggests using
SOURCE retention, so no change is necessary there.

Change-Id: I223c1d7fd3a3b4d0b7f8acf8394d6ec2327748d4
1 file changed