[LangRef] improve documentation of SNaN in the default FP environment

Make it explicit that SNaN is not handled differently than
QNaN in the LLVM default floating-point environment.

Note that an IEEE-754-compliant model disallows transforms
like "X * 1.0 -> X". That is because math operations are
expected to convert SNaN to QNaN (set the signaling bit).

But LLVM has had those kinds of transforms from the beginning:
https://alive2.llvm.org/ce/z/igb55y

We should be IEEE-754-compliant under strict-FP (the logic is
implemented with a helper named canIgnoreSNaN()), but I don't
think there is any demand to do that with default optimization.

See issue #43070 for earlier draft/discussion about this change.

Differential Revision: https://reviews.llvm.org/D143074