submitting-changes.chapter.md: explain that purple arrows are manual

The documentation for this diagram explains that the blue arrows are
automatic processes which happen every six hours.  There is no
explanation about how the purple arrows happen or how often.

As a new contributor to nixpkgs, I incorrectly assumed that the purple
arrows were also automatic processes (they aren't), which left me sort
of confused about what the whole scheme was accomplishing.

Recently I went through the github history to see how often these
events happen, and realized that the purple arrows are (a) triggered
manually by a nixpkgs project member and (b) happen much, much, much
less frequently than every six hours.

Now everything makes a lot more sense.  I suggest the wording change
in this commit, or something similar, to save future contributors the
same confusion that I experienced.

doc/contributing/submitting-changes.chapter.md

@@ -227,7 +227,7 @@ digraph {
 }
 ```
 
-[This GitHub Action](https://github.com/NixOS/nixpkgs/blob/master/.github/workflows/periodic-merge-6h.yml) brings changes from `master` to `staging-next` and from `staging-next` to `staging` every 6 hours.
+[This GitHub Action](https://github.com/NixOS/nixpkgs/blob/master/.github/workflows/periodic-merge-6h.yml) brings changes from `master` to `staging-next` and from `staging-next` to `staging` every 6 hours; these are the blue arrows in the diagram above.  The purple arrows in the diagram above are done manually and much less frequently.  You can get an idea of how often these merges occur by looking at the `git-log` for `master` and searching for `from NixOS/staging-next` (for the upper purple arrow) or `staging-next into staging` (for the lower arrow).
 
 
 ### Master branch {#submitting-changes-master-branch}