hclsyntax: Clarify Body.EndRange comment

[radeksimko]

As discussed via Slack this field is no longer (and probably never was) used for reporting missing items.

Instead Body has a dedicated method for this:

hcl/hclsyntax/structure.go

Lines 281 to 287 in 2ebbb5d

	func (b *Body) MissingItemRange() hcl.Range {
	return hcl.Range{
	Filename: b.SrcRange.Filename,
	Start: b.SrcRange.Start,
	End: b.SrcRange.Start,
	}
	}

[apparentlymart]

We did have at least a few Terraform releases where this comment was true, but we switched how hclsyntax.Body implements MissingItemRange after realizing that the start of the body (which appears on the same line as the block definition the body belongs to) gives better context for a problem when we highlight it as part of a diagnostic message and so indeed this comment is stale now.