RFR: 8280001: Serial: Add documentation to heap memory layout

[Albert Mingkun Yang]

On Fri, 14 Jan 2022 08:26:04 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Comments-only change of adding some documentation.
> 
> Test: build

Thanks for the review.

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PR: https://git.openjdk.java.net/jdk/pull/7075