Tile grid documentation improvements

Using the term 'bottom-left' for origin and origins is misleading, because
many developers use -y-1 for the tile url's y in their tile url functions,
and the origin really only determines where tile coordinates start to
increase from left to right and from bottom to top.

externs/olx.js

@@ -6148,7 +6148,9 @@ olx.tilegrid.TileGridOptions.prototype.minZoom;
 
 
 /**
- * Origin, i.e. the bottom-left corner of the grid. Default is null.
+ * The tile grid origin, i.e. where the `x` and `y` axes meet (`[z, 0, 0]`).
+ * Tile coordinates increase from left to right and from bottom to top. Default
+ * is null.
  * @type {ol.Coordinate|undefined}
  * @api stable
  */
@@ -6156,9 +6158,10 @@ olx.tilegrid.TileGridOptions.prototype.origin;
 
 
 /**
- * Origins, i.e. the bottom-left corners of the grid for each zoom level. If
+ * Tile grid origins, i.e. where the `x` and `y` axes meet (`[z, 0, 0]`), for
- * given, the array length should match the length of the `resolutions` array,
+ * each zoom level. If given, the array length should match the length of the
- * i.e. each resolution can have a different origin.
+ * `resolutions` array, i.e. each resolution can have a different origin. Tile
+ * coordinates increase from left to right and from bottom to top.
  * @type {Array.<ol.Coordinate>|undefined}
  * @api stable
  */