alloc: trivial top level comments for linear/stack

alloc/linear.hpp

@@ -21,6 +21,8 @@
 
 
 namespace util { namespace alloc {
+    // allocate progressively across a buffer without concern for deallocation.
+    // deallocation is a noop; the only way to free allocations is via reset.
     class linear {
     public:
         linear (const linear&) = delete;

alloc/stack.hpp

@@ -22,6 +22,8 @@
 
 
 namespace util { namespace alloc {
+    // allocate memory from a buffer in a stacklike manner. deallocation that
+    // is not correctly ordered has undefined (read 'bad') results.
     class stack {
     public:
         stack (const stack&) = delete;