Document rule for attachment names for proper functioning in embeds #5535

Job	Run time
MSRV	50s
Test (macOS)	1m 42s
Test (nightly)	3m 19s
Test (stable)	3m 27s
Test (beta)	3m 6s
Build docs	40s
Test (Windows)	5m 3s
Examples	3m 50s
Test (native-tls)	2m 45s
Test (no default features)	44s
Check minimal versions	35s
Test (no cache)	2m 58s
Test (no gateway)	1m 44s
Test (unstable Discord API features)	3m 8s
Test (simd-json)	3m 9s
Test (chrono, chrono)	48s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	1m 11s
Test (builder without model, builder, true)	29s
Test (unstable Discord API (no default features), unstable_discord_api, true)	24s
	39m 52s

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