docs(core): Polish API docs for `Metadata` #8357

Job	Run time
build-cpp-doc	24s
build-c-doc	19s
build-dav-server-opendalfs-doc	47s
build-fuse3-opendal-doc	44s
build-haskell-doc	2m 25s
build-java-doc	19s
build-lua-doc	11s
build-nodejs-doc	25s
build-object-store-opendal-doc	43s
build-ocaml-doc	5m 0s
build-opendal-compat-doc	41s
build-parquet-opendal-doc	42s
build-python-doc	3m 49s
build-rust-doc	3m 41s
build-unftp-sbe-opendal-doc	1m 2s
build-virtiofs-opendal-doc	46s
build-website	1m 41s
	23m 39s

Provide feedback