docs: mention that clobbering output regs in inline asm is illegal.
docs: mention that clobbering output regs in inline asm is illegal.

I've found this out the hard way; LLVM will not normally catch this
error (unless -verify-machineinstrs is passed), and under certain
very specific circumstances (such as register scavenger running
under pressure) this would result in an opaque crash in codegen.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@280071 91177308-0d34-0410-b5e6-96231b3b80d8

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