docs(core): Polish API docs for `Metadata` #2249

Job	Run time
build_all_features	4m 53s
build_all_platforms (ubuntu-latest)	2m 54s
build_under_wasm	41s
build_default_features (ubuntu-latest)	35s
build_all_platforms (macos-latest)	2m 28s
check_clippy	3m 55s
build_default_features (macos-latest)	37s
check_docs	3m 27s
build_default_features (windows-latest)	2m 15s
build_all_platforms (windows-latest)	6m 31s
check_msrv	30s
unit	6m 35s
	35m 21s

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