Rollup merge of rust-lang#25602 - parkr:patch-1, r=alexcrichton

Padding and alignment are often not implemented by types and can cause confusion in the user.  Per discussion with @alexcrichton, here is my PR.

/cc time-rs/time#98

Author: steveklabnik

src/libcollections/fmt.rs

@@ -353,6 +353,10 @@
 //! * `^` - the argument is center-aligned in `width` columns
 //! * `>` - the argument is right-aligned in `width` columns
 //!
+//! Note that alignment may not be implemented by some types. A good way
+//! to ensure padding is applied is to format your input, then use this
+//! resulting string to pad your output.
+//!
 //! ## Sign/#/0
 //!
 //! These can all be interpreted as flags for a particular formatter.