Clarify how String::leak and into_boxed_str differ

Author: jonhoo

library/alloc/src/string.rs

@@ -1940,8 +1940,10 @@ impl String {
 
     /// Converts this `String` into a <code>[Box]<[str]></code>.
     ///
-    /// This will drop any excess capacity.
+    /// Before doing the conversion, this method discards excess capacity like [`shrink_to_fit`].
+    /// Note that this call may reallocate and copy the bytes of the string.
     ///
+    /// [`shrink_to_fit`]: String::shrink_to_fit
     /// [str]: prim@str "str"
     ///
     /// # Examples
@@ -1967,10 +1969,10 @@ impl String {
     /// this function is ideally used for data that lives for the remainder of the program's life,
     /// as dropping the returned reference will cause a memory leak.
     ///
-    /// It does not reallocate or shrink the `String`,
-    /// so the leaked allocation may include unused capacity that is not part
-    /// of the returned slice. If you don't want that, call [`into_boxed_str`],
-    /// and then [`Box::leak`].
+    /// It does not reallocate or shrink the `String`, so the leaked allocation may include unused
+    /// capacity that is not part of the returned slice. If you want to discard excess capacity,
+    /// call [`into_boxed_str`], and then [`Box::leak`] instead. However, keep in mind that
+    /// trimming the capacity may result in a reallocation and copy.
     ///
     /// [`into_boxed_str`]: Self::into_boxed_str
     ///