[mdatagen] Add docs for optional events (#13131) #30860

Job	Run time
setup-environment	41s
govulncheck	1m 29s
checks	2m 49s
lint	5m 15s
unittest-matrix (ubuntu-latest, ~1.23)	6m 31s
unittest-matrix (ubuntu-latest, ~1.24)	6m 44s
test-coverage	11m 21s
cross-build-collector (linux, 386)	58s
cross-build-collector (darwin, amd64)	49s
cross-build-collector (linux, arm64)	48s
cross-build-collector (darwin, arm64)	52s
cross-build-collector (linux, s390x)	48s
cross-build-collector (linux, arm, 7)	45s
cross-build-collector (aix, ppc64)	52s
cross-build-collector (windows, 386)	53s
cross-build-collector (windows, amd64)	55s
cross-build-collector (linux, ppc64le)	56s
cross-build-collector (linux, amd64)	50s
unittest	1s
	44m 17s

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