docs: hint how to ignore spans if very short

[SimonSimCity]

PR for #1701 - adding a hint to the performance tips showing how to ignore spans having a short duration.

Needs to be checked by a native English speaker ...

Checklist

• Update documentation
• Commit message follows commit guidelines

[watson]

Thanks a lot for the PR 💯I just left a few review comments

[bmorelli25]

Thanks so much @SimonSimCity! Once @watson's recommendations have been implemented I'll kick off the docs ci to ensure everything builds nicely :)

[SimonSimCity]

Thanks, I've just updated the pull-request.

Please also take the time to comment to my other two pull-requests, where it is about code I'd like to have in.

[watson]

Just a tiny little change with a missing ---- to open the code block so that it renders correctly. Otherwise all good 👍

[bmorelli25]

@elasticmachine, run elasticsearch-ci/docs

[bmorelli25]

Thank you! I'm running a final doc build. If that passes, I think we're good to go!

[bmorelli25]

Docs preview LGTM. Thank you again!

[bmorelli25]

@SimonSimCity it looks like the approved-contributor check is failing. Can you please sign the contributor agreement with the same email address you used for these commits?

[SimonSimCity]

@bmorelli25 I've done this already last week (see: #1724 (comment))

[ghost]

💚 Build Succeeded

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Build stats

• Build Cause: [Started by user Thomas Watson, Replayed Don't rely on native database client binaries #9]

• Start Time: 2020-05-11T07:33:28.009+0000

• Duration: 3 min 8 sec (187707)

• Commit: 011d321

[bmorelli25]

Assigning myself so this doesn't get lost. @watson, can I merge this even if the Travis CI build fails? I'll merge master to run tests again.