Another doc patch: it's define-attr, not define-attribute. Other minor gotchas.

[Hans-Peter Nilsson]

	* doc/rtl.texi: Fix typo: define-attr, not define-attribute.
	(Enumerated constants): Mention that an ifield must not specify a
	multi-ifield.
	(Instruction operands): Ditto for index.
	(Expressions) <parallel>: Remove misplaced mention of local
	variables.
	<if>: Mention that mode must be specified and non-VOID when the
	result is used.

Approved by FChE and committed.

brgds, H-P