driver: Don't highlight in isearch

zsh version 5.2 and lower don't support ISEARCHMATCH_ACTIVE and
we are unable to re-apply zle_highlight on top. Therefore it is
impossible to see the underlined matched area.

Since that information is more important, completely disable
highlighting in isearch in that case.

To do that, we need to make sure we are actually called when
something changes in isearch.

Trumps #257.

The FAQ entry presupposes #245 will be fixed (in time for the release) too.

README.md

@@ -32,6 +32,24 @@ custom widgets have been created (i.e., after all `zle -N` calls and after
 running `compinit`).  Widgets created later will work, but will not update the
 syntax highlighting.
 
+### Why does syntax highlighting not work while searching history?
+
+_This problem is fixed in zsh 5.3 and newer._
+
+Highlighting the command line during an incremental history search
+(with the `history-incremental-search-backward` widget, which is
+bound by default to <kbd>Ctrl+R</kbd> in zsh's emacs keymap) requires zsh 5.3
+or newer.
+
+Under zsh 5.2 and older, the zsh-default underlining of the matched portion
+of the buffer remains available, but zsh-syntax-highlighting's additional
+highlighting is unavailable.  (Those versions of zsh do not provide enough
+information to allow computing the highlighting correctly.)
+
+See [issue #288][i288] for details.
+
+[i288]: https://github.com/zsh-users/zsh-syntax-highlighting/pull/288
+
 ### How are new releases announced?
 
 There is currently no "push" announcements channel.  However, the following