docs: Polish docs for write with if not exists #7998

Job	Run time
build-c-doc	27s
build-nodejs-doc	25s
build-java-doc	17s
build-python-doc	3m 8s
build-rust-doc	5m 20s
build-cpp-doc	46s
build-dav-server-opendalfs-doc	48s
build-fuse3-opendal-doc	41s
build-lua-doc	12s
build-haskell-doc	3m 24s
build-ocaml-doc	5m 42s
build-object-store-opendal-doc	1m 25s
build-opendal-compat-doc	44s
build-parquet-opendal-doc	49s
build-unftp-sbe-opendal-doc	1m 11s
build-virtiofs-opendal-doc	40s
build-website	1m 48s
	27m 47s

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