RFR: Add notes on avoiding spurious introduction of labels.

[Stuart Marks]

On Fri, 16 Oct 2020 18:18:46 GMT, Iris Clark <iris at openjdk.org> wrote:

>> Include discussion of using label autocomplete to ensure that an existing label is used.
>
> Yes!
> 
> Historical Note: The use of labels has changed over time.  In very early versions of the JDK bug system, the equivalent
> labels field was treated as an index.  Because all of that information has been retained, the set of existing labels is
> huge!

@JesperIRL Can you sponsor this change? (If you're ok with it of course.) Iris has reviewed it so I think it falls to
you to sponsor it. Thanks.

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PR: https://git.openjdk.java.net/guide/pull/31