std: Make mem's doc slightly more accurate

Author: brson

src/libstd/mem.rs

@@ -8,7 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-//! Functions relating to memory layout
+//! Basic functions for dealing with memory
+//!
+//! This module contains functions for querying the size and alignment of
+//! types, initializing and manipulating memory.
 
 #[allow(missing_doc)]; // FIXME