Change the typedefs and docs for updateTag to match how FXP actually behaves

docs/v4/2.XMLparseOptions.md

@@ -907,7 +907,11 @@ Output
 **Note**: Unpaired tag can't be used as closing tag e.g. `</unpaired>` is not allowed.
 ## updateTag
 
-This property allow you to change tag name when you send different name, skip a tag from the parsing result when you return false, or to change attributes.
+This method allows you to modify the tag by changing the tag name, modifying its attributes, or completely skip the tag from the parsing result.
+* When a string is returned that will be the new tag name.
+* Returning `false` or `undefined` will skip this tag from the parsing result.
+* Returning `true` will make no changes at all
+
 ```js
 const xmlDataStr = `
     <rootNode>
@@ -930,4 +934,17 @@ const parser = new XMLParser(options);
 const output = parser.parse(xmlDataStr);
 ```
 
+Output
+```json
+{
+    "rootNode": {
+        "A": {
+            "#text": "value",
+            "At": "Home"
+        },
+        "At": "Home"
+    }
+}
+```
+
 [> Next: XmlBuilder](./3.XMLBuilder.md)

src/fxp.d.ts

@@ -36,10 +36,12 @@ Control how tag value should be parsed. Called only if tag value is not empty
 Change the tag name when a different name is returned. Skip the tag from parsed result when false is returned. 
 Modify `attrs` object to control attributes for the given tag.
 
-@returns {string} new tag name.
-@returns false to skip the tag
+@returns {string} to change the tag name
+@returns {true} to not make any changes
+@returns {false} to skip the tag from the parsing result
+@returns {undefined} to skip the tag from the parsing result
    */
-  updateTag: (tagName: string, jPath: string, attrs: {[k: string]: string}) =>  string | boolean;
+  updateTag: (tagName: string, jPath: string, attrs: {[k: string]: string}) =>  string | boolean | undefined;
 };
 type strnumOptions = {
   hex: boolean;