fix: allow XOR and XNOR gates to have more than 2 inputs

[cynthi8]

Motivation for this change

There have been several requests for XOR and XNOR gates to allow more than 2 inputs [#376] [1] [2]. This PR fixes that and updates the docs.

As far as I can see, the previous concern was a potentially ambiguous IEC symbol [ = 1 ]. However, extended gates are widely interpreted to represent the cascade of such binary operations and there is clear demand to display such symbols (from me too).

As a weird quirk, ICE XNOR gates have always been able to accept more than 2 inputs - in opposition to the docs.

Fixes #376

Checklist

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• Code changes are licensed under GPLv2 + LPPLv1.3c
• Documentation changes are licensed under FDLv1.2