ICU: Document that collation rules are omitted.

The collation rules normally returned by RuleBasedCollator.getRules()
are omitted in Android to save space:

https://android.googlesource.com/platform/external/icu/+/9cf98b3

This additional Javadoc comment was previously added:

https://android.googlesource.com/platform/libcore/+/1c13b45

But then it got lost again when the entire source file was replaced:

https://android.googlesource.com/platform/libcore/+/6c0e55c

This commit reinstates a comment about omitted collation rules.

Bug: 27441320
(cherry picked from commit a09dc30505bef70e0e6eb8afa886a59864e25fd2)

Change-Id: I535cee0f0b2c3926ede319b6bb285f68008487cc