Update events.md with time period limit documentation

Kyle Lemonrequested to merge
Jklemon17/gitlab-ce:event-api-time-limit-docs into master

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

Adding documentation for the fact that the Events api does not include events older than one year, since they are removed from the Events table.

Resolves issue #35912 (closed)

Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

  • Make sure the old link is not removed and has its contents replaced with a link to the new location.
  • Make sure internal links pointing to the document in question are not broken.
  • Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ directory.
  • Make sure to add redirect_from to the new document if there are any Disqus comments on the old document thread.
  • If working on CE, submit an MR to EE with the changes as well.
  • Ping one of the technical writers for review.

From: @Jklemon17 @PolarisTLX

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