Add documentation for the GPX format.

src/ol/format/gpxformat.exports

@@ -1,3 +1,5 @@
 @exportSymbol ol.format.GPX
 @exportProperty ol.format.GPX.prototype.readFeature
 @exportProperty ol.format.GPX.prototype.readFeatures
+@exportProperty ol.format.GPX.prototype.readProjection
+@exportProperty ol.format.GPX.prototype.writeFeatures

src/ol/format/gpxformat.js

@@ -363,6 +363,16 @@ ol.format.GPX.WPT_PARSERS_ = ol.xml.makeParsersNS(
     });
 
 
+/**
+ * Read the first feature from a GPX source.
+ *
+ * @function
+ * @param {ArrayBuffer|Document|Node|Object|string} source Source.
+ * @return {ol.Feature} Feature.
+ */
+ol.format.GPX.prototype.readFeature;
+
+
 /**
  * @inheritDoc
  */
@@ -384,6 +394,16 @@ ol.format.GPX.prototype.readFeatureFromNode = function(node) {
 };
 
 
+/**
+ * Read all features from a GPX source.
+ *
+ * @function
+ * @param {ArrayBuffer|Document|Node|Object|string} source Source.
+ * @return {Array.<ol.Feature>} Features.
+ */
+ol.format.GPX.prototype.readFeatures;
+
+
 /**
  * @inheritDoc
  */
@@ -407,6 +427,15 @@ ol.format.GPX.prototype.readFeaturesFromNode = function(node) {
 };
 
 
+/**
+ * Read the projection from a GPX source.
+ *
+ * @param {ArrayBuffer|Document|Node|Object|string} source Source.
+ * @return {ol.proj.Projection} Projection.
+ */
+ol.format.GPX.prototype.readProjection;
+
+
 /**
  * @inheritDoc
  */
@@ -770,6 +799,16 @@ ol.format.GPX.V1_1 = function() {
 goog.inherits(ol.format.GPX.V1_1, ol.format.GPX);
 
 
+/**
+ * Encode an array of features in the GPX format.
+ *
+ * @function
+ * @param {Array.<ol.Feature>} features Features.
+ * @return {ArrayBuffer|Node|Object|string} Result.
+ */
+ol.format.GPX.prototype.writeFeatures;
+
+
 /**
  * @inheritDoc
  */