Fix INumber.Sign documentation for zero return value

[Copilot]

The documentation for INumber<T>.Sign method incorrectly stated that when the input value is zero, it returns INumberBase<TSelf>.Zero. However, the actual behavior is that it returns the integer value 0 (Int32).

Changes Made

• Updated the return value description in xml/System.Numerics/INumber1.xmlto correctly state that zero inputs return0instead of referencingINumberBase.Zero`

Before

<returns>A positive value if <paramref name="value" /> is positive, <see cref="P:System.Numerics.INumberBase`1.Zero" /> if <paramref name="value" /> is zero, and a negative value if <paramref name="value" /> is negative.</returns>

After

<returns>A positive value if <paramref name="value" /> is positive, <c>0</c> if <paramref name="value" /> is zero, and a negative value if <paramref name="value" /> is negative.</returns>

This aligns with the method's actual return type of System.Int32 and matches the behavior documented in the code examples showing that Math.Sign() returns integer values (0, 1, -1).

Fixes #11207.

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