[Fleet] Improve text strings in new agent binary download section

[Supplementing]

Summary

Closes #139151

Changed placeholder and help text to be more descriptive (using approved text from issue).

Checklist

Check the PR satisfies following conditions.

Reviewers should verify this PR satisfies this list as well.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
• Flaky Test Runner was used on any tests changed
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines

Identify risks

n/a

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[nchaulet]

@Supplementing when deleting a translation key, you will have to delete the related translations, this could be done automatically by running node scripts/i18n_check.js --fix in your kibana directory

[elasticmachine]

💔 Build Failed

• Buildkite Build
• Commit: 6150c0d

Failed CI Steps

• Jest Tests #9
• Jest Tests #9

Test Failures

• [job] [logs] Jest Tests #9 / EditOutputFlyout should render the flyout if there is no download source provided
• [job] [logs] Jest Tests #9 / EditOutputFlyout should render the flyout if there is no download source provided

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
fleet	1.7MB	1.7MB	-82.0B

History

• 💔 Build #262551 failed dac914c
• 💔 Build #262495 failed 7c48667