[libc++][docs] Update `[[nodiscard]]` coding guidelines (#178384)
as requested in
https://github.com/llvm/llvm-project/pull/176936#pullrequestreview-3706775444
Clarifies when not to apply `[[nodiscard]]`.
https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant
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Co-authored-by: Nikolas Klauser <nikolasklauser@berlin.de>
Co-authored-by: Hristo Hristov <zingam@outlook.com>