Document invariants for the MessageCache #1125

Job	Run time
Test (macOS)	1m 10s
Test (stable)	1m 12s
Build docs	20s
Test (beta)	1m 21s
Examples	3m 6s
Test (Windows)	2m 33s
Check minimal versions	27s
Test (native-tls)	1m 6s
Test (no default features)	46s
MSRV	0s
Test (no cache)	1m 50s
Test (nightly)	1m 19s
Test (no gateway)	1m 22s
Test (unstable Discord API features)	1m 17s
Test (zlib compression)	1m 18s
Test (zstd compression)	1m 14s
Test (zlib and zstd compression)	1m 13s
Test (chrono, chrono)	43s
Test (unstable API + typesize, default unstable typesize, true)	53s
Test (builder without model, builder, true)	26s
Test (unstable Discord API (no default features), unstable, true)	27s
	24m 3s

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