build

doc: add comment to explain the syntax of folded examples #14669

Job	Run time
build (ubuntu-latest)	4m 27s
Build jj-lib without Git support	43s
build (macos-13)	10m 58s
build (macos-14)	3m 45s
cargo-deny (advisories)	23s
build (windows-latest)	9m 34s
cargo-deny (bans licenses sources)	26s
build (ubuntu-latest, --all-features)	5m 27s
Check protos	25s
Clippy check	2m 35s
Check that MkDocs can build the docs	21s
Check that MkDocs can build the docs with Poetry 1.8	22s
Check formatting	16s
	39m 42s

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