[llvm] [LangRef] Document the fourth operand for vp.cttz/ctlz. NFC (PR #97387)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon approved this pull request.

LGTM - with one minor peeve - the subsection is called "Arguments:" but then we refer to them as operands :\

https://github.com/llvm/llvm-project/pull/97387