RFR: 8276549: Improve documentation about ContainerPtr encoding

[Thomas Schatzl]

Hi all,

  can I have reviews for this trivial? change to add some more documentation to `G1CardSet::ContainerPtr`?

Testing: local compilation

Thanks,
  Thomas

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Commit messages:
 - fix whitespace
 - 8276549: Improve documentation about ContainerPtr encoding

Changes: https://git.openjdk.java.net/jdk/pull/8811/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8811&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276549
  Stats: 25 lines in 1 file changed: 20 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8811.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8811/head:pull/8811

PR: https://git.openjdk.java.net/jdk/pull/8811