[lld] [lld/ELF] Add documentation on large sections (PR #82560)

[Reid Kleckner via llvm-commits]

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rnk wrote:

I think I drew this on a whiteboard originally, and the intention of the arcs was to signal that data above the large data threshold from the original small data section would move to the large data section, keeping overall binary size the same, but changing the layout to make the small section portion of the binary more compact.

Maybe it would be better expressed with two colors (blue/red), where blue indicates small data (below the threshold) and red indicates large data (above the threshold), and the .l-prefixed sections are all red.

That was the idea, but let's not make that diagram a requirement.

https://github.com/llvm/llvm-project/pull/82560