CONTRIBUTE: More formally specify the commit description

CONTRIBUTE: More formally specify the commit description

This codifies what people have actually used in git commits over the
past 6 years. I've left off some lesser-used headers that appear to
duplicate others and tried to describe a consistent use for several
others that were used more arbitrarily.

This makes it easier for new committers to find out the kinds of things
we want to acknowledge, makes it easier to perform statistical analysis
on commits, and opens the possibility of performing lint checks on
descriptions before submission.

Reviewed-by: Daniel Stenberg
Reviewed-by: Jay Satiro
Closes #10478