Improvements to PR template (#13198)

I like a bit more of a hint what the PR is supposed to achieve.

Also, I took out a suggestion not to write tests.  We can decide
case-by-case if it's ok not to add a test; no need to encourage it.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,4 +1,8 @@
 <!--
+    Please give your PR a title in the form "area: short description".  For example "tsdb: reduce disk usage by 95%"
+
+    If your PR is to fix an issue, put "Fixes #issue-number" in the description.
+
     Don't forget!
 
     - Please sign CNCF's Developer Certificate of Origin and sign-off your commits by adding the -s / --signoff flag to `git commit`. See https://github.com/apps/dco for more information.
@@ -7,8 +11,6 @@
 
     - Where possible use only exported APIs for tests to simplify the review and make it as close as possible to an actual library usage.
 
-    - No tests are needed for internal implementation changes.
-
     - Performance improvements would need a benchmark test to prove it.
 
     - All exposed objects should have a comment.