Update `sig_stfl.h` document for #1650 #1233

Job	Run time
Check code formatting	1m 1s
Check upstream code is properly integrated	1m 0s
Check that code passes a basic build before starting heavier tests	1m 0s
linux_arm_emulated (armhf, armhf, -DOQS_ENABLE_SIG_SPHINCS=OFF -DOQS_USE_OPENSSL=OFF -DOQS_OPT_TA...	33m 0s
linux_intel (alpine, openquantumsafe/ci-alpine-amd64:latest, -DOQS_STRICT_WARNINGS=ON -DOQS_USE_O...	9m 8s
linux_intel (alpine-openssl-all, openquantumsafe/ci-alpine-amd64:latest, -DOQS_STRICT_WARNINGS=ON...	18m 24s
linux_intel (alpine-noopenssl, openquantumsafe/ci-alpine-amd64:latest, -DOQS_STRICT_WARNINGS=ON -...	11m 1s
linux_intel (focal-nistr4-openssl, openquantumsafe/ci-ubuntu-focal-x86_64:latest, -DOQS_STRICT_WA...	4m 2s
linux_intel (jammy-std-openssl3, openquantumsafe/ci-ubuntu-jammy:latest, -DOQS_STRICT_WARNINGS=ON...	3m 25s
linux_intel (address-sanitizer, openquantumsafe/ci-ubuntu-focal-x86_64:latest, -DCMAKE_C_COMPILER...	12m 48s
	1h 34m 49s

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