feat: added examples of API usage

there are examples of some of these methods in the codebase for the agent, I moved those here too

https://github.com/newrelic/node-newrelic/blob/main/api.js

src/content/docs/apm/agents/nodejs-agent/api-guides/nodejs-agent-api.mdx

@@ -251,6 +251,18 @@ Use these API calls to [expand your instrumentation with custom instrumentation]
     You <DoNotTranslate>**must**</DoNotTranslate> handle custom transactions manually by calling `newrelic.getTransaction()` at the start of your transaction, and then call `transaction.end()` when you are finished. New Relic begins timing the transaction when `newrelic.startWebTransaction()` is called and ends the transaction when `transaction.end()` is called.
 
     You can also return a promise to indicate the end of the transaction. Please note that if this promise rejects, it does not automatically hook into New Relic's error tracking. This needs to be done manually with [`noticeError()`](#noticeError).
+
+    Example:
+    ```js
+    var newrelic = require('newrelic')
+    newrelic.startWebTransaction('/some/url/path', function() {
+      var transaction = newrelic.getTransaction()
+      setTimeout(function() {
+        // do some work
+        transaction.end()
+      }, 100)
+    })
+    ```
   </Collapser>
 
   <Collapser
@@ -272,6 +284,18 @@ Use these API calls to [expand your instrumentation with custom instrumentation]
     You <DoNotTranslate>**must**</DoNotTranslate> handle custom transactions manually by calling `newrelic.getTransaction()` at the start of your transaction, and then call `transaction.end()` when you are finished. New Relic begins timing the transaction when `newrelic.startBackgroundTransaction()` is called and ends the transaction when `transaction.end()` is called.
 
     You can also return a promise to indicate the end of the transaction. Please note that if this promise rejects, it does not automatically hook into New Relic’s error tracking. This needs to be done manually with [`noticeError()`](#noticeError).
+
+  Example:
+  ```js
+  var newrelic = require('newrelic')
+  newrelic.startBackgroundTransaction('Red October', 'Subs', function() {
+    var transaction = newrelic.getTransaction()
+    setTimeout(function() {
+      // do some work
+      transaction.end()
+    }, 100)
+  })
+  ```
   </Collapser>
 
   <Collapser
@@ -314,6 +338,14 @@ Use these API calls to [expand your instrumentation with custom instrumentation]
     * The optional `callback` is a function passed to the handler to fire after its work is done.
 
     The agent begins timing the segment when `startSegment` is called. The segment is ended when either the `handler` finishes executing, or `callback` is fired, if it is provided.
+
+    Example:
+    ```js
+    newrelic.startSegment('mySegment', false, function handler() {
+      // The returned promise here will signify the end of the segment.
+      return myAsyncTask().then(myNextTask)
+    })
+    ```
   </Collapser>
 </CollapserGroup>
 
@@ -833,7 +865,17 @@ New Relic's Node.js agent includes additional API calls.
     }
     ```
 
-    Calling this function multiple times will replace previously defined versions of this callback function.
+    Calling this function multiple times will replace previously defined versions of this callback function. Example:
+
+    ```js
+    function myCallback(metadata) {
+      if (metadata['http.statusCode'] === '400') {
+        return 'Bad User Input'
+      }
+    }
+
+    newrelic.setErrorGroupCallback(myCallback)
+    ```
 
     To learn more about this function, please refer to our [example app](https://github.com/newrelic/newrelic-node-examples/tree/main/error-fingerprinting).
   </Collapser>