Fix inaccurate documentation in member function mutability.

[entropylost]

It previously stated that changes would simply not be applied, which is not the case based on the playground:

struct Foo {
    int count;
    void setCount(int x) {
        count = x;
    }
}

void test() {
    Foo f;
    f.setCount(1);
}

Gives this compile error

USER error: /user.slang(4): error 30011: left of '=' is not an l-value.
        count = x;
              ^
/user.slang(4): note 30049: a 'this' parameter is an immutable parameter by default in Slang; apply the `[mutating]` attribute to the function declaration to opt in to a mutable `this`
(0): error 39999: import of module 'user' failed because of a compilation error
(0): fatal error 39999: compilation ceased

It may also be worth removing the "for gpu performance considerations" since it doesn't impact performance at all.

[CLAassistant]

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