* Correct some prototypes and macros with respect to "const"ness.

* Add the extra macros and examples due to the lh_doall[_arg] modifications
  made recently. The existing example is also reworked for consistency.

* Rewrite, tweak, and supplement bits of the existing comments that seemed
  (IMHO) to be a little convoluted and misleading.

* Add a NOTE section that explains the use of macros and avoiding function
  casts (ie. generate a wrapper as with the macros, or prototype any
  callback functions exactly to not require casting). Also, explain the
  "const" approach taken in LHASH for the purposes of API comprehensibility
  and also application code auditing.