Document memory allocation APIs

Add some docs where they were missing,
attempt to fix them where they were out of date.
diff --git a/src/libstd/alloc.rs b/src/libstd/alloc.rs
index 9904634..9126155 100644
--- a/src/libstd/alloc.rs
+++ b/src/libstd/alloc.rs
@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-//! dox
+//! Memory allocation APIs
 
 #![unstable(issue = "32838", feature = "allocator_api")]