DOC: Clarify centroid subtlety

[pllim]

Description

This pull request is to clarify centroid subtlety that it requires background subtraction to be accurate.

Related PRs:

• Imviz: Allow Subset recentering via centroiding #1823
• No more centroiding in Simple Aperture Photometry plugin #1841

Change log entry

• Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts,
  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)? 🐱

[codecov]

Codecov Report

Base: 88.29% // Head: 88.29% // No change to project coverage 👍

Coverage data is based on head (5f7a96d) compared to base (e841613).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1854   +/-   ##
=======================================
  Coverage   88.29%   88.29%           
=======================================
  Files          95       95           
  Lines       10396    10396           
=======================================
  Hits         9179     9179           
  Misses       1217     1217           

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[rosteen]

LGTM!

[pllim]

@larrybradley , you happy with the wordings now?

[larrybradley]

LGTM