src: describe what NODE_MODULE_VERSION is for

Current comment described what to do with it when the ABI changes, but
implied that node would load modules with newer ABI numbers, which it
will not.

src/node_version.h

@@ -44,6 +44,9 @@
       (minor) == NODE_MINOR_VERSION && (patch) <= NODE_PATCH_VERSION))
 
 /**
+ * Node.js will refuse to load modules that weren't compiled against its own
+ * module ABI number, exposed as the process.versions.modules property.
+ *
  * When this version number is changed, node.js will refuse
  * to load older modules.  This should be done whenever
  * an API is broken in the C++ side, including in v8 or