Update docs w.r.t. warning and strictness options.

Update docs w.r.t. warning and strictness options.

* doc/automake.texi (Strictness): Document that some warnings are
turned off by default in `foreign' strictness.
(Options): Divide into new sections "Options generalities" and
"List of Automake options".  Fix typo (colon instead of full
stop).  Document option precedence (AUTOMAKE_OPTIONS wins over
AM_INIT_AUTOMAKE which wins over command line).  Also document
interactions between options specifying strictness and those
specifying warnings.