doc: clarify N-API version Matrix

I was asked by a community member if they needed to
recompile for v12.x based on the version matrix. Add
additional context to help ensure it is clear that this
is not the case.

PR-URL: https://github.com/nodejs/node/pull/27942
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Christopher Hiller <boneskull@boneskull.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/n-api.md

@@ -145,6 +145,12 @@ available to the module code.
 
 ## N-API Version Matrix
 
+N-API versions are additive and versioned independantly from Node.js.
+Version 4 is an extension to version 3 in that it has all of the APIs
+from version 3 with some additions. This means that you
+do not need to recompile for new versions of Node.js which are
+listed as supporting a later version.
+
 |       | 1       | 2        | 3        | 4        |
 |:-----:|:-------:|:--------:|:--------:|:--------:|
 | v6.x  |         |          | v6.14.2* |          |