merged_tree: slightly adjust doc comment of inner trees() helpers #16752

Job	Run time
build (ubuntu-latest)	5m 4s
Build jj-lib without Git support	46s
build (macos-13)	10m 15s
Check protos	35s
build (macos-14)	3m 51s
build (windows-latest)	9m 14s
Check that MkDocs can build the docs	13s
Check that MkDocs can build the docs with latest Python and uv	15s
build (ubuntu-latest, --all-features)	6m 4s
Check formatting	16s
Clippy check	2m 34s
cargo-deny (advisories)	23s
cargo-deny (bans licenses sources)	22s
	39m 52s

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