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

[Mo-Gul]

Motivation for this change

Checklist

Please signoff your commits to explicitly state your agreement to the Developer Certificate of Origin. If that is not possible you may check the boxes below instead:

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

---

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