Add warning that matching behavior may change at any time #435

Job	Run time
Alpine Linux	2m 28s
FreeBSD-x86_64 with rust stable	54s
Linux-x86_64 with rust stable	1m 14s
Linux-x86_64 (rustls-native-roots) with rust stable	1m 30s
Linux-x86_64 (native-tls) with rust stable	1m 21s
Linux-x86_64 (native-tls-vendored) with rust stable	2m 43s
Linux-aarch64 with rust stable	1m 10s
Linux-arm with rust stable	1m 16s
Linux-i686 with rust stable	58s
Linux-powerpc with rust stable	1m 2s
Linux-powerpc64 with rust stable	47s
Linux-powerpc64le with rust stable	47s
Linux-riscv64 with rust stable	1m 6s
Linux-s390x with rust stable	47s
NetBSD-x86_64 with rust stable	2m 4s
Windows-aarch64 with rust stable	1m 23s
Windows-i686 with rust stable	1m 33s
Windows-x86_64 with rust stable	3m 15s
macOS-x86_64 with rust stable	1m 49s
macOS-aarch64 with rust stable	1m 22s
Linux-x86_64 with rust beta	1m 13s
Linux-x86_64 with rust nightly	1m 31s
	32m 13s

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