* 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.
