Added docs for onEmit and offEmit Symbols (#27)

* Added docs for onEmit and offEmit Symbols

* Removed not needed `apis.json` file

* Improved docs to include better example

docs/symbols-offEmit.md

@@ -0,0 +1,32 @@
+@typedef {function(function(*))} can-symbol/symbols/offEmit can.offEmit
+@parent can-symbol/symbols/observe
+@description Defines how observables can stop listening to the object's value being emitted.
+
+@signature `@@can.offEmit( handler(newValue) )`
+
+The `@@@@can.offEmit` symbol points to a function that unregisters 
+ `handler` from being called when the object's value
+ is emitted.  
+
+```js
+const obj = function(value) {
+	if(arguments.length >= 1) {
+		obj.currentValue = value;
+		obj.handlers.forEach(function(handler){
+			handler.call(obj, value);
+		});
+	} else {
+		return obj.currentValue;
+	}
+};
+
+obj[canSymbol.for("can.offEmit")] = function(handler){
+	if(!obj.handlers) {
+		obj.handlers = [];
+	}
+	obj.handlers.splice(obj.handlers.indexOf(handler), 1);
+}
+```
+
+@this {*} any object with a mutable value
+@param {function(this:*, *)} handler(newValue) The handler was previously added with [can-symbol/symbols/onEmit `@@can.onEmit`]. 

docs/symbols-onEmit.md

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+@typedef {function(function(*))} can-symbol/symbols/onEmit can.onEmit
+@parent can-symbol/symbols/observe
+@description Defines how observables can listen to the object's value whenever it is emitted.
+
+@signature `@@can.onEmit( handler(newValue) )`
+
+The `@@@@can.onEmit` symbol points to a function that registers 
+ `handler` to be called back with the new value of the object when it
+ is emitted. `@@@@can.onEmit` can be used when you wish to emit a value that isn't changing.
+
+```js
+const obj = function(value) {
+	if(arguments.length >= 1) {
+		obj.currentValue = value;
+		obj.handlers.forEach(function(handler){
+			handler.call(obj, value);
+		});
+	} else {
+		return obj.currentValue;
+	}
+};
+
+obj[canSymbol.for("can.onEmit")] = function(handler){
+	if(!obj.handlers) {
+		obj.handlers = [];
+	}
+	obj.handlers.push(handler);
+}
+
+const listener = function (value) {
+	console.log('emitted', value);
+}
+
+obj[canSymbol.for("can.onEmit")](listener);
+
+obj(5); // -> output: "emitted 5"
+obj(5); // -> output: "emitted 5"
+```
+
+@this {*} any object with a mutable value
+@param {function(this:*, *)} handler(newValue) The handler must be called back with `this` as the instance of the observable.