docs(apk): document apkindex methods (#1393) #2495

Job	Run time
source-date-epoch	1m 43s
build-date-epoch	1m 36s
build-nginx-multiarch	1m 17s
build-nginx-all-arches (x86_64)	1m 17s
annotations	1m 7s
build-nginx-all-arches (386)	1m 2s
build-nginx-all-arches (armv7)	1m 6s
build-nginx-all-arches (aarch64)	1m 11s
build-nginx-all-arches (riscv64)	1m 5s
build-nginx-all-arches (s390x)	1m 9s
build-nginx-all-arches (ppc64le)	1m 5s
build-all-examples-amd64 (ubuntu-latest)	2m 20s
build-all-examples-amd64 (macos-latest)	1m 59s
	17m 57s

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