Document behaviour of StatePrep getting absorbed by simulators

[jzaia18]

Context:
Initial StatePreps being automatically handled without applying the gate on various simulators is not a documented feature.

Description of the Change:
Updates StatePrep docs to explicitly mention that some simulators may remove and automatically handle an initial StatePrep.

A separate PR to Catalyst will also stop this behaviour for null.qubit specifically.

Benefits:

Possible Drawbacks:

Related GitHub Issues:
#8650

See also PennyLaneAI/catalyst#2195

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.42%. Comparing base (fba4729) to head (265c119).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8651   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files         587      587           
  Lines       61969    61969           
=======================================
  Hits        61613    61613           
  Misses        356      356           

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