[Doc] Explain flowR subgraph and slicing criteria convenience toggles in tests

[EagleoutIce]

This eases the way we write dataflow tests and so others writing tests should know!
This is linked to #1104.

The following test showcases the potential, we no longer have to specify the complete graph!

flowr/test/functionality/dataflow/processing-of-elements/expression-lists/dataflow-read-tests.ts

Lines 144 to 154 in 77183c4

	const distractor = 'x <- 3\ny <- 4\nz <- 2\n';
	assertDataflow(label('without distractors', [...OperatorDatabase['<-'].capabilities, 'numbers', 'name-normal', 'newlines', 'name-escaped']),
	shell, '`a` <- 2\na',
	emptyGraph()
	.use('2@a')
	.reads('2@a', '1@`a`'),
	{
	expectIsSubgraph: true,
	resolveIdsAsCriterion: true
	}
	);