Document sentinel type in langref

[marler8997]

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[andrewrk]

docs removed in b3cbf29

this content should go here:

zig/lib/std/builtin.zig

Lines 148 to 151 in 170de5e

	/// The type of the sentinel is the element type of the pointer, which is
	/// the value of the `child` field in this struct. However there is no way
	/// to refer to that type here, so we use `var`.
	sentinel: var,