docs: Explain why `ConstF64` is not PartialEq #3149

Job	Run time
Check for changes in Python files	3s
check python 3.10	16s
Check serialization schema	12s
check python 3.13	23s
Build HUGR binary	36s
test python 3.10	42s
test python 3.13	25s
Required checks 🐍	1s
	2m 38s

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