docs: improve public-api plugin docs (#11951) #30

Job	Run time
build (ubuntu-latest, linux_openresty, lua-resty-worker-events, t/http3/admin)	13m 21s
build (ubuntu-latest, linux_openresty, lua-resty-worker-events, t/plugin/[a-k]*)	35m 19s
build (ubuntu-latest, linux_openresty, lua-resty-worker-events, t/plugin/[l-z]*)	32m 51s
build (ubuntu-latest, linux_openresty, lua-resty-worker-events, t/admin t/cli t/config-center-yam...	25m 2s
build (ubuntu-latest, linux_openresty, lua-resty-worker-events, t/node t/pubsub t/router t/script...	26m 2s
build (ubuntu-latest, linux_openresty, lua-resty-events, t/http3/admin)	13m 31s
build (ubuntu-latest, linux_openresty, lua-resty-events, t/plugin/[a-k]*)	34m 16s
build (ubuntu-latest, linux_openresty, lua-resty-events, t/plugin/[l-z]*)	33m 16s
build (ubuntu-latest, linux_openresty, lua-resty-events, t/admin t/cli t/config-center-yaml t/con...	25m 27s
build (ubuntu-latest, linux_openresty, lua-resty-events, t/node t/pubsub t/router t/script t/secr...	25m 39s
	4h 24m 44s

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