Add clarification for FMV behaviour.

This clarifies cases like:

```C
__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);
}
```

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

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

main/acle.md

@@ -2696,10 +2696,12 @@ following:
   in one of the translation units.
   * Implicitly, without this attribute,
   * or explicitly providing the `default` in the attribute.
-* All instances of the versions shall share the same function
-  signature and calling convention.
+* All instances of the versions shall share the same calling convention 
+* All function declarations shall share compatible function signatures 
+  that are combined to a single signature for all versions.
 * All the function versions must be declared at the translation
   unit in which the definition of the default version resides.
+* All function declarations must be at file scope level.
 
 The attribute `__attribute__((target_clones("name",...)))` expresses the
 following: