clarify getRawServiceInternal 'retry' docs am: e5f8f7c02c am: ec1e232221 am: 70ecfbf092 am: dbebcc1af2

Original change: https://android-review.googlesource.com/c/platform/system/libhidl/+/1656229

Change-Id: I5383603bc5f8b519df4d20e4e394eb5cec08e8f8