Revise and somewhat expand `Prepare` docs #5553

Job	Run time
cargo-deny-advisories	31s
cargo-deny	32s
check-blocking	3s
check-packetline (ubuntu-latest)	5s
lint	3m 44s
pure-rust-build	2m 8s
test	10m 59s
test-32bit (i386)	5m 56s
test-fast (windows-latest)	16m 26s
test-fixtures-windows	15m 32s
test-32bit (arm32v7)	4m 11s
test-fast (macos-latest)	2m 59s
test-journey	4m 0s
test-fast (ubuntu-latest)	3m 43s
test-fast (ubuntu-22.04-arm)	3m 51s
WebAssembly (wasm32-unknown-unknown)	1m 3s
WebAssembly (wasm32-wasip1)	35s
Tests pass	1s
	1h 16m 19s

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