wp_memory.w improvement

Minor revision to list traversal. Clarify sentinel role of wp_txn_head.

- Improve documentation of Diagnostic Aliases
- Change memory transaction log to refer to cicular doubly-linked list
- Annotate degenerate WP_LIST_PREPEND macro (to be extracted at a later
  date)
- Change verbiage of Allocation and Deallocation
- Correct list traversal in wp_free_wrapper
- Correct list traversal in wp_memory_get_diagnostics
- Correct list traversal in wp_memory_print_stats
- Rename several section names
- Fix spelling