Start documenting new [rewrite_strat] tactic that applies rewriting
according to a given strategy.
- Enhance the hints/lemmas strategy to support "using tac" comming from
rewrite hints to solve side-conditions.  


git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16558 85f007b7-540e-0410-9357-904b9bb8a0f7