cli doc: opm serve does not detect DC changes after startup

[joelanford]

Description of the change:
A CLI documentation change for opm serve

Motivation for the change:
Clarify that opm serve will not reload served content when underlying files in the served directory change.

See operator-framework/olm-docs#177 (comment)

Reviewer Checklist

• Implementation matches the proposed design, or proposal is updated to match implementation
• Sufficient unit test coverage
• Sufficient end-to-end test coverage
• Docs updated or added to /docs
• Commit messages sensible and descriptive

[timflannagan]

/lgtm

[codecov]

Codecov Report

Merging #750 (208da74) into master (a4fd56c) will not change coverage.
The diff coverage is n/a.

❗ Current head 208da74 differs from pull request most recent head b8041af. Consider uploading reports for the commit b8041af to get more accurate results

@@           Coverage Diff           @@
##           master     #750   +/-   ##
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  Coverage   50.24%   50.24%           
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  Files         101      101           
  Lines        8691     8691           
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  Hits         4367     4367           
  Misses       3471     3471           
  Partials      853      853           

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[kevinrizza]

/approve

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

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