docs: add comment explaining application and release vsn discrepancy

src/emqx.app.src

@@ -1,6 +1,11 @@
 {application, emqx,
  [{id, "emqx"},
   {description, "EMQ X"},
+  %% Note: this version is not the same as the release version!  This
+  %% is simply the emqx `application' version, which is separate from
+  %% the emqx `release' version, which in turn is comprised of several
+  %% apps, one of which is this.  See `emqx_release.hrl' for more
+  %% info.
   {vsn, "4.3.14"}, % strict semver, bump manually!
   {modules, []},
   {registered, []},