(NFC) PULL_REQUEST_TEMPLATE.md - Improve suggestions for under-the-hood/non-UI changes

(NFC) PULL_REQUEST_TEMPLATE.md - Improve suggestions for under-the-hood/non-UI changes

Overview
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The "pull-request template" is presented to a developer when submitting a
proposed change; it prompts them to fill in additional information.  This
revision aims to make the prompts more useful in the common case where change
does *not* involve the user-interface.

Before
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The `PULL_REQUEST_TEMPLATE.md` suggests that the "Before" and "After" should
be demonstrated with screenshots.

This is wonderful for UI-oriented changes.

However, if a change is focused under-the-hood (APIs, data-structures,
class-hierarchies, refactorings, etc) - then the prompt is kind of
meaningless/non-applicable.  A meaningless prompt understandably elicits
weak responses like "*Before: Some incorrect metadata*" or "*After: It's
less confusing*".

After
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The `PULL_REQUEST_TEMPLATE.md` still recommends using concrete example in
"Before"/"After", but it's more open about what that means.

For a UI-oriented change, it means screenshots.  For an
API/technical-service change, it means code-snippets.