docs: add info about sync interval

[morey-tech]

Add information about the sync interval to the Automated Sync Policy page. Users will likely check this page when looking for how to configure the sync interval.

Signed-off-by: Nicholas Morey nicholas@morey.tech

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Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• Optional. My organization is added to USERS.md.
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[codecov]

Codecov Report

Base: 45.64% // Head: 45.65% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (a0acead) compared to base (41b1ebe).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #11281   +/-   ##
=======================================
  Coverage   45.64%   45.65%           
=======================================
  Files         239      239           
  Lines       28991    28996    +5     
=======================================
+ Hits        13233    13238    +5     
  Misses      13939    13939           
  Partials     1819     1819           

Impacted Files	Coverage Δ
pkg/apis/application/v1alpha1/types.go	55.17% <0.00%> (+0.32%)	⬆️

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