docs: jwe-decrypt secret length must be 32 chars #15993

Job	Run time
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[a-k]*)	31m 7s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[l-z]*)	29m 15s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/admin t/cli t/config-center-yaml...	21m 23s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/node t/pubsub t/router t/script ...	21m 57s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[a-k]*)	29m 13s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[l-z]*)	29m 48s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/admin t/cli t/config-center-yaml t/cont...	21m 52s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/node t/pubsub t/router t/script t/secre...	21m 26s
	3h 26m 1s

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