Checkstyle: check for proper sentence ending of JavaDoc @param/@return/etc

We recently had various instances in reviews where missing .s were noted, such as:

• !1046 (comment 2813221)
• !1040 (comment 2747334)

And there was a question why Checkstyle doesn't check for this:

• !1040 (comment 2747393)

We can check for it with Checkstyle, using a custom regex.