docs: document the integer encoding used by CodePcGroup and PcGroupCode #8002

Job	Run time
Cross compiler for s390x	34m 8s
Test in OpenBSD	8m 17s
testinstall-bare - - ubuntu-latest	5m 21s
testbugfix - CONFIGFLAGS="--enable-memory-checking" - ubuntu-latest	30m 24s
testpackages testinstall-loadall - - ubuntu-latest	33m 36s
makemanuals testmakeinstall - CONFIGFLAGS="--prefix=/tmp/gapprefix" NO_COVERAGE=1 - ubuntu-latest	8m 16s
testmockpkg testinstall - - windows-2022	22m 55s
testmockpkg testinstall testworkspace - BOOTSTRAP_MINIMAL=yes - macos-latest	11m 13s
testmanuals - - ubuntu-latest	5m 46s
testinstall - JULIA=yes CONFIGFLAGS="--enable-debug" - ubuntu-22.04	7m 51s
testexpect testmockpkg testspecial test-compile testlibgap testkernel testworkspace - - ubuntu-latest	8m 19s
testinstall - HPCGAP=yes ABI=64 - ubuntu-latest	11m 1s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=32 BUILDDIR=out-of-tree CONFIGFLAGS="" - ubuntu-22.04	4m 43s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=64 BUILDDIR=out-of-tree CONFIGFLAGS="--enable-valgrind" - ubuntu-latest	5m 33s
teststandard - - ubuntu-latest	17m 2s
Finalize Codecov notifications	8s
Send Slack notification on status change	0s
	3h 34m 33s

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