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

[Mo-Gul]

Motivation for this change

Checklist

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---

Remaining instances are

pgf/tex/generic/pgf/graphdrawing/lua/pgf/gd/doc/ogdf/energybased/FMMMLayout.lua

Lines 108 to 123 in 0280303

	example
	[[
	\tikz {
	\graph [FMMMLayout, node sep=1cm] { subgraph C_n[n=6]; };
	\draw [red, ultra thick, |-|] (1.south) -- ++(down:1cm);
	}
	]]
	example
	[[
	\tikz {
	\graph [FMMMLayout, node sep=5mm] { subgraph C_n[n=6]; };
	\draw [red, ultra thick, |-|] (1.south) -- ++(down:5mm);
	}
	]]

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 ...

[hmenke]

I'm not sure whether I merged the support for this new syntax yet. I have to check that before merging.

[Mo-Gul]

Yes, you had. At least it seems to work in the currently available first example of

pgf/tex/generic/pgf/graphdrawing/lua/pgf/gd/force/ControlElectric.lua

Lines 42 to 60 in 0280303

	examples = {
	{
	options = [["preamble={\usetikzlibrary{graphs,graphdrawing} \usegdlibrary{force}}"]],
	code = [["
	\tikz \graph [spring electrical layout, horizontal=0 to 1]
	{ 0 [electric charge=1] -- subgraph C_n [n=10] };
	"]]
	},{
	code = [["
	\tikz \graph [spring electrical layout, horizontal=0 to 1]
	{ 0 [electric charge=5] -- subgraph C_n [n=10] };
	"]]
	},{
	code = [["
	\tikz \graph [spring electrical layout, horizontal=0 to 1]
	{ [clique] 1 [electric charge=5], 2, 3, 4 };
	"]]
	}
	}

which shows as

https://pgf-tikz.github.io/pgf/pgfmanual.pdf#page=477

[hmenke]

On 2026-06-15 a more comprehensive testsuite was introduced for PGF/TikZ. This tests all new contributions against the examples extracted from the manual and checks whether anything has changed to avoid regressions. Since this pull request was opened before that, these tests were not run automatically. I have run l3build check on the result of the merge of this pull request branch into master. The result can be found below.

Tip

Your pull request might have failed some tests. This does not mean that your contribution will be rejected. In fact, depending on the changes you propose, it might even be expected that some tests fail, however, this needs to be evaluated carefully.

All tests passed