Add docstring for `ions_decomposition` pass #11421

Job	Run time
Determine runner type to use / determine_workflow_runner	1s
Set build matrix / set-constants	7s
Catalyst-Runtime Build (gcc)	56s
Runtime Code Coverage (Linux)	1m 33s
LLVM Build (gcc)	33s
Enzyme Build (gcc)	15s
MHLO Dialect Build (gcc)	6s
Runtime Tests (Linux) (lightning, gcc)	22s
Runtime Tests (Linux) (lightning-kokkos, gcc)	31s
Runtime Tests (Linux) (openqasm, gcc)	20s
Runtime Tests (Linux) (oqc, gcc)	48s
Runtime Tests (Linux) (oqd, gcc)	31s
Quantum Dialects Build (gcc)	4m 48s
Frontend Tests (backend="lightning.kokkos") (gcc)	6m 51s
Frontend Tests (backend="openqasm3") (gcc)	1m 51s
Frontend Tests (gcc)	8m 46s
	28m 19s

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