constants.xml: add dots at the end of sentences

[mmalferov]

[Girgias]

This needs a rebase to fix the conflicts

[cmb69]

I'm not quite sure about this. Few if any of these are proper sentences, and as such terminating with a full-stop wouldn't be quite right, although there is at least one occurrence where this is already done. I suggest to either convert the bullet items to full sentences, or to remove the full-stops altogether (maybe replacing them with semicolons).

cc @kamil-tekiela