Remove incorrect note from string_add_assign docs
Remove incorrect note from string_add_assign docs

The docs claim that `String::push_str` is better than `String::add` because `String::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
graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust