docs: Explain why `ConstF64` is not PartialEq #3156

Job	Run time
Check for changes	6s
Build benchmarks 🏋️	2m 29s
check	1m 50s
tests (Rust nightly, coverage)	4m 11s
tests (Rust 1.75)	3m 44s
tests (Rust stable, all features)	2m 51s
tests (Rust beta)	3m 24s
tests (Rust stable, no features)	2m 30s
tests hugr-llvm	0s
Check standard extensions	18s
Required checks 🦀	1s
	21m 24s

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