Remove incorrect note from string_add_assign docs

[mbrubeck]

The docs claim that String::push_str is better than String::add because add allocates a new string and drops the old one, but this is not true. In fact, add reuses the existing string and grows it only if its capacity is exceeded, exactly like push_str. Their performance is identical since add is just a wrapper for push_str:

    fn add(mut self, other: &str) -> String {
        self.push_str(other);
        self
    }

https://github.com/rust-lang/rust/blob/35bf1ae25799a4e62131159f052e0a3cbd27c960/src/liballoc/string.rs#L1922-L1925

[phansch]

thanks!