docs: specify ThreadSafeRepository is not Send/Sync without "parallel" #5543

Job	Run time
lint	3m 40s
pure-rust-build	2m 1s
test	10m 17s
cargo-deny-advisories	28s
test-32bit (i386)	5m 46s
test-fast (windows-latest)	14m 27s
test-32bit (arm32v7)	4m 3s
test-fixtures-windows	15m 15s
test-journey	3m 48s
test-fast (macos-latest)	2m 59s
test-fast (ubuntu-latest)	4m 2s
test-fast (ubuntu-22.04-arm)	3m 59s
cargo-deny	27s
check-blocking	6s
check-packetline (ubuntu-latest)	6s
WebAssembly (wasm32-unknown-unknown)	1m 5s
WebAssembly (wasm32-wasip1)	39s
Tests pass	1s
	1h 13m 9s

Provide feedback