Add clarification for FMV behaviour.

[AlfieRichardsArm]

This clarifies cases like:

__attribute__((target_version("default")))
int foo(int (* x)[12]){
    return (*x)[11];
}

__attribute__((target_version("sve")))
int foo(int (* x)[10]){
    return (*x)[7];
}

int bar()
{
    int y[10];
    return foo(&y);
}

int __attribute__((target_version("default"))) fn (int x) {
    return 1;
}

void bar () {
    int __attribute__((target_version("sve2"))) fn (int);
    fn(1);
}

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