Improve explanation of lifetimes #7015

Job	Run time
find-languages	4s
bare-metal (src/bare-metal/alloc-example, aarch64-unknown-none)	44s
format	36s
cargo (ubuntu-latest)	31s
bare-metal (src/bare-metal/aps/examples, aarch64-unknown-none)	46s
Translation diff	2m 26s
cargo (macos-latest)	43s
bare-metal (src/bare-metal/microcontrollers/examples, thumbv7em-none-eabihf)	50s
bare-metal (src/exercises/bare-metal/compass, thumbv7em-none-eabihf)	55s
typos	3s
bare-metal (src/exercises/bare-metal/rtc, aarch64-unknown-none)	41s
build (ar)	8m 25s
build (bn)	5m 51s
build (da)	6m 36s
build (de)	6m 32s
build (el)	7m 41s
build (en)	4m 50s
build (es)	5m 33s
build (fa)	6m 23s
build (fr)	5m 40s
build (id)	7m 41s
build (it)	5m 43s
build (ja)	7m 33s
build (ko)	7m 4s
build (pl)	6m 6s
build (pt-BR)	6m 15s
build (ro)	5m 48s
build (ru)	5m 41s
build (tr)	5m 43s
build (uk)	5m 43s
build (vi)	5m 50s
build (zh-CN)	6m 39s
build (zh-TW)	7m 42s
	2h 29m 18s

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