[docs] remove weird distinctions/disclaimer in the API reference (#256)

This prose makes it sound like the doc is only for advanced users or for adding auto-instrumentation, and it's somehow different than "using" the beeline.  Let's clear that up - it's a reference for the API and not meant to be the place you go for usage examples.

docs/API.md

@@ -1,6 +1,6 @@
-**This document is intended for anyone looking to write custom instrumentation for your favorite NPM package.**
+This document is intended as a reference to the Beeline for Node.js API.
 
-If you're looking to understand how to **use** the Beeline for Node.js in your application,
+If you're looking for examples of how to use the Beeline in your application,
 please check out our [Usage and Examples](https://docs.honeycomb.io/beeline/nodejs/) documentation instead.
 
 ## Accessing the API