RFR: 8248877: Document API contract for MetaspaceObj subtypes

[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 ....).

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

Commit messages:
 - 8248877: Document API contract for MetaspaceObj subtypes

Changes: https://git.openjdk.java.net/jdk/pull/4327/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4327&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248877
  Stats: 18 lines in 1 file changed: 18 ins; 0 del; 0 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