RFR: 8272975: ParallelGC: add documentation to heap memory layout

[Albert Mingkun Yang]

Simple change of adding some documentation.

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Commit messages:
 - heap-doc

Changes: https://git.openjdk.java.net/jdk/pull/5247/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5247&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272975
  Stats: 20 lines in 1 file changed: 20 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5247.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5247/head:pull/5247

PR: https://git.openjdk.java.net/jdk/pull/5247