docs: Polish docs for write with if not exists #2036

Job	Run time
build_all_features	5m 15s
build_all_platforms (ubuntu-latest)	2m 38s
build_default_features (ubuntu-latest)	36s
build_all_platforms (macos-latest)	3m 1s
build_default_features (macos-latest)	45s
build_under_wasm	1m 9s
build_all_platforms (windows-latest)	5m 57s
build_default_features (windows-latest)	2m 28s
check_clippy	4m 30s
check_docs	3m 30s
check_msrv	34s
unit	6m 51s
	37m 14s

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