doc: fix -W and %expect documentation some.

* NEWS (2.4.3): Mention that there are documentation fixes.
* doc/bison.texinfo (Expect Decl): Make it clear that %expect
turns conflicts into errors not warnings.
(Shift/Reduce): Likewise.
(Bison Options): Don't mention -Wsyntax.  It doesn't exist.
(cherry picked from commit 3ffdd5f1254fd9403a5a1d31dbdc5b2756c22775)