Improve explanation for safety of MapCompact autocorrection in docs

[Splines]

When I first read about the Performance/MapCompact cop, I didn't understand the following sentence at first glance:

This cop's autocorrection is unsafe because map { ... }.compact that is not compatible with filter_map.

• The that in this sentence is confusing me grammatically-wise.
• And this explanation did not hint me to why it's not safe. In hindsight, I don't know why I haven't just looked at the example one line above to see the actual difference. But maybe, for other users, it might be as well beneficial to improve the wording and refer to the given example to clearly point out that falsy values are filtered out by filter_map as well (see the Ruby docs). Compare that with compact, which only gets rid of nil values.

This fixes #280. The issue was already closed, however, it "fixes" it in the sense that the underlying confusion should be mitigated by a better wording in the docs.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists). -> ⚠ I've included the related issue it in the PR message as well as in the changelog entry. I forgot to add it to the commit message itself, hope that's ok.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together. -> ⚠ The first two commits of this PR could be squashed together. As I'm not rebasing very often, I'd have to look up how that works. If these two commits (that probably get squashed together into master anyways), do not impose a serious problem, maybe I can keep them as they are?
• Added tests. -> ⚠ Not needed as this is just a small documentation change
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

🛑 As a sidenote: Note that in the last checklist item, you're linking to the contribution guide of rubocop (via the changelog entry format link), not that of this repo.

[koic]

Once you have addressed the comments, please squash the commits into one.

[Splines]

Thanks for your review. I addressed all your points.

Note that I ran bundle exec rake default again and 8 examples failed in rspec. After that, I executed the command again and now everything is passing and RuboCop did not detect any offenses. After yet another execution of that command, still everything works as expected ;)

[koic]

Thanks!