RFR: 8272439: G1: add documentation to G1CardSetInlinePtr

[Albert Mingkun Yang]

Add some documentation to `class G1CardSetInlinePtr`, and simplify a field initialization in `G1CardSetConfiguration`.

Test: hotspot_gc

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Commit messages:
 - cardset

Changes: https://git.openjdk.java.net/jdk/pull/5108/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5108&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272439
  Stats: 34 lines in 3 files changed: 33 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5108.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5108/head:pull/5108

PR: https://git.openjdk.java.net/jdk/pull/5108