RFR: 8248877: Document API contract for MetaspaceObj subtypes [v2]

[Ioi Lam]

> This PR adds documentation for the (non-virtual) functions that subtypes of MetaspaceObj are required to implement in order for certain templates to work properly. 
> 
> Requested by @rose00 (sorry I took a long time ....).

Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:

  @coleenp comments; clean up

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4327/files
  - new: https://git.openjdk.java.net/jdk/pull/4327/files/b0db2f36..f5b83fb0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4327&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4327&range=00-01

  Stats: 10 lines in 1 file changed: 1 ins; 4 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4327.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4327/head:pull/4327

PR: https://git.openjdk.java.net/jdk/pull/4327