Document CodeMirror.countColumn

Issue codemirror#4929

Author: marijnh

doc/manual.html

@@ -2229,6 +2229,9 @@ <h3 id="api_static">Static properties</h3>
       various <a href="#event_change">event handlers</a>). The
       returned position will be the end of the changed
       range, <em>after</em> the change is applied.</dd>
+
+      <dt id="countColumn"><code><strong>CodeMirror.countColumn</strong>(line: string, index: number, tabSize: number) → number</code></dt>
+      <dd>Find the column position at a given string index using a given tabsize.</dd>
     </dl>
 </section>