Some doc fixes and additions for spin operators

[leburgel]

• Adds explicit statement that σⁱ = 2 ⋅ Sⁱ to the docstrings of all the Pauli operator aliases, as well as a reference to the matching spin operator. The convention to obtain Pauli operators from spin operators was only mentioned in the manual page, making the Pauli operator docstrings quite useless by themselves.
• Fixes mismatches in the docstrings of the Q-state Potts model where the method definition used a small q as a keyword argument while the docstring showed a capital Q keyword, as well as some math formatting.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/operators/spinoperators.jl	92.22% <100.00%> (ø)

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