doc: update process.versions.modules documentation

This commit adds a description of `process.versions.modules`,
based on the comment in `src/node_version.h` lines 47-50.

PR-URL: https://github.com/nodejs/node/pull/9901
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/process.md

@@ -1683,7 +1683,9 @@ added: v0.2.0
 * {Object}
 
 The `process.versions` property returns an object listing the version strings of
-Node.js and its dependencies.
+Node.js and its dependencies. In addition, `process.versions.modules` indicates
+the current ABI version, which is increased whenever a C++ API changes. Node.js
+will refuse to load native modules built for an older `modules` value.
 
 ```js
 console.log(process.versions);