Document overrides of clone_from()

Specifically, when an override doesn't just forward to an inner type,
document the behavior and that it's preferred over simply assigning
a clone of source. Also, change instances where the second parameter is
"other" to "source".

library/std/src/path.rs

@@ -1628,6 +1628,10 @@ impl Clone for PathBuf {
         PathBuf { inner: self.inner.clone() }
     }
 
+    /// Clones the contents of `source` into `self`.
+    ///
+    /// This method is preferred over simply assigning `source.clone()` to `self`,
+    /// as it avoids reallocation if possible.
     #[inline]
     fn clone_from(&mut self, source: &Self) {
         self.inner.clone_from(&source.inner)