Add documentation of recent serialize_none_as_null attribute, and bum… #250

Job	Run time
Test Individual Features (binary)	5s
Build (ubuntu-latest, x86_64-unknown-linux-gnu)	9s
Test Individual Features (json)	10s
Build (ubuntu-latest, x86_64-pc-windows-gnu)	23s
Test Individual Features (ron)	10s
Test (ubuntu-latest, x86_64-unknown-linux-gnu)	16s
Test Individual Features (toml)	5s
Test Individual Features NoStd (binary)	14s
Build (ubuntu-latest, wasm32-unknown-unknown)	9s
Test Individual Features NoStd (json)	14s
Build (macos-latest, x86_64-apple-darwin)	15s
Test No Std (ubuntu-latest, x86_64-unknown-linux-gnu)	23s
Test Individual Features NoStd (ron)	14s
Build (windows-latest, x86_64-pc-windows-msvc)	35s
Test Individual Features NoStd (toml)	10s
Rustfmt (ubuntu-latest, x86_64-unknown-linux-gnu)	5s
	3m 37s

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