extend.texi (Extended Asm): Add blurb about using Explicit Reg Vars to enforce register allocation with...

	* doc/extend.texi (Extended Asm): Add blurb about using Explicit
	Reg Vars to enforce register allocation with general constraints.
	(Explicit Reg Vars): Clarify relation to asm statements.
	(Local Reg Vars): Similar.

From-SVN: r88265