subgroup: fix docs and broadcast const variant #1123

Job	Run time
Compiletest (macOS-latest)	9m 16s
Test (windows-2022)	19m 6s
Test (macOS-latest)	16m 17s
Android (ubuntu-24.04, aarch64-linux-android)	8m 59s
Test (ubuntu-24.04)	10m 56s
Compiletest (ubuntu-24.04)	6m 58s
Compiletest (windows-2022)	15m 2s
Lint	6m 13s
Difftest (windows-2022)	27m 37s
Difftest (ubuntu-24.04)	15m 27s
Difftest (macOS-latest)	21m 34s
cargo-deny	1m 4s
test_success	2s
	2h 38m 31s

Provide feedback