RFR: 8248877: Document API contract for MetaspaceObj subtypes

[Ioi Lam]

On Thu, 3 Jun 2021 00:36:56 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> 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 ....).

I cleaned up the comments per Coleen's comments. I also removed the "debug only" comment about on_stack() since in most cases they are inline functions and will be elided by the compiler anyway.

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