runtime: document gctrace output in relation to MemStats

[JohnRusk]

How do the heap sizes output by gctrace map to the various values of MemStats? I assume that the heap values from gctrace roughly match one (or two?) of the MemStats fields, but I can't find anything that documents that.

I see in the code that the gctrace outputs are mostly based on heap_live and the MemStats values are not (directly) based on the the same thing... but I'm guessing there's an indirect or approximate link. Knowing such a mapping would help a lot in using gctrace, since it doesn't have heaps of documentation but MemStats has lots.

[seankhliao]

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[JohnRusk]

@seankhliao In that case, please re-open this as a documentation bug. It should be documented.

[seankhliao]

cc @aclements @randall77 @mknyszek @prattmic

[mknyszek]

I'm hoping to do something around documentation of how the GC views memory and the heap in the 1.18, so I've assigned it to myself. I'll leave it in the backlog just in case, but it's on my radar.