RFR: 8272439: G1: add documentation to G1CardSetInlinePtr [v2]

[Albert Mingkun Yang]

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

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5108/files
  - new: https://git.openjdk.java.net/jdk/pull/5108/files/e282d72d..1b441153

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5108&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5108&range=00-01

  Stats: 4 lines in 2 files changed: 2 ins; 1 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