docs: correct/add `subgraph` preamble comments (fixes #1194)

[Mo-Gul]

Motivation for this change

Checklist

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• [ ] Code changes are licensed under GPLv2 + LPPLv1.3c
• [x] Documentation changes are licensed under FDLv1.2

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Remaining instances are

https://github.com/pgf-tikz/pgf/blob/0280303d03594e1ea13f95f116234a803591b4af/tex/generic/pgf/graphdrawing/lua/pgf/gd/doc/ogdf/energybased/FMMMLayout.lua#L108-L123

but since these are not part of the current manual and I couldn't make this part of the manual run, I decided not to fix them.

[Mo-Gul]

sorry, missed one file ...