Use CanvasLineJoin type instead of string

src/ol/style/Stroke.js

@@ -9,7 +9,7 @@
  * See {@link module:ol/color~Color} and {@link module:ol/colorlike~ColorLike} for possible formats.
  * Default null; if null, the Canvas/renderer default black will be used.
  * @property {string} [lineCap='round'] Line cap style: `butt`, `round`, or `square`.
- * @property {string} [lineJoin='round'] Line join style: `bevel`, `round`, or `miter`.
+ * @property {CanvasLineJoin} [lineJoin='round'] Line join style: `bevel`, `round`, or `miter`.
  * @property {Array<number>} [lineDash] Line dash pattern. Default is `undefined` (no dash).
  * Please note that Internet Explorer 10 and lower do not support the `setLineDash` method on
  * the `CanvasRenderingContext2D` and therefore this option will have no visual effect in these browsers.
@@ -61,7 +61,7 @@ class Stroke {
 
     /**
      * @private
-     * @type {string|undefined}
+     * @type {CanvasLineJoin|undefined}
      */
     this.lineJoin_ = options.lineJoin;
 
@@ -134,7 +134,7 @@ class Stroke {
 
   /**
    * Get the line join type for the stroke.
-   * @return {string|undefined} Line join.
+   * @return {CanvasLineJoin|undefined} Line join.
    * @api
    */
   getLineJoin() {
@@ -208,7 +208,7 @@ class Stroke {
   /**
    * Set the line join.
    *
-   * @param {string|undefined} lineJoin Line join.
+   * @param {CanvasLineJoin|undefined} lineJoin Line join.
    * @api
    */
   setLineJoin(lineJoin) {