Fix split_layers description in CollectCliffords

[josemmiranda04]

Fix #16833

It uupdates the split_layers argument description in CollectCliffords to clarify that collected blocks are split into layers of non-overlapping instructions, in other words, depth-1 sub-blocks.

Validation:

• python -m compileall qiskit/transpiler/passes/optimization/collect_cliffords.py

AI/LLM disclosure

• No part of this submission is LLM generated.
• Some written text was generated by: Claude
• Some submitted code was generated by:

[CLAassistant]

All committers have signed the CLA.

[jakelishman]

Please confirm you have accurately filled out the LLM disclosure.

[josemmiranda04]

Please confirm you have accurately filled out the LLM disclosure.

Already accurately filled. Thank you for the feedback!

[jakelishman]

I see that there is a box ticked, but the writing is highly suggestive that an LLM authored the entire PR as well as the description, which is why I asked.

[coveralls]

Coverage Report for CI Build 33162310476

Coverage increased (+0.004%) to 87.747%

Details

• Coverage increased (+0.004%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• 11 coverage regressions across 2 files.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

11 previously-covered lines in 2 files lost coverage.

File	Lines Losing Coverage	Coverage
crates/qasm2/src/parse.rs	6	97.63%
crates/qasm2/src/lex.rs	5	92.03%

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Coverage Stats

Relevant Lines:	131079
Covered Lines:	115018
Line Coverage:	87.75%
Coverage Strength:	982413.25 hits per line

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💛 - Coveralls