CONTRIBUTING.md: Clarify the PR description section

[easwars]

This change adds more details to the "PR description" section and hopefully makes it easier for contributors to add PR titles and description that meets our requirements.

RELEASE NOTES: n/a

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.96%. Comparing base (e8ad657) to head (0ea02fd).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8592      +/-   ##
==========================================
+ Coverage   81.91%   81.96%   +0.04%     
==========================================
  Files         415      415              
  Lines       40694    40694              
==========================================
+ Hits        33336    33355      +19     
+ Misses       5968     5956      -12     
+ Partials     1390     1383       -7     

see 18 files with indirect coverage changes

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[dfawley]

As discussed offline, I am weary to add 20 lines to this already long document. Maybe we can solve our problems by providing a better template that includes the fields we want to see in the order we want to see them? E.g.

Fixes|Updates: #<issue>

A description of the change being made.  If fixing a bug, please include a brief description of the bug and/or how it is triggered.  Include any necessary explanation of alternatives considered/etc.

All changes must include either:

RELEASE NOTES:
* <component>: <user-understandable explanation of the change being made>

OR for non-user-visible changes:

RELEASE NOTES: n/a