docs: add documentation for rfl::Commented feature #508

Job	Run time
windows-msvc-shared (AVRO)	6m 56s
windows-msvc-shared (TOML)	3m 59s
windows-msvc-shared (JSON)	10m 37s
windows-msvc-shared (XML)	3m 50s
windows-msvc-shared (YAML)	4m 38s
windows-msvc-shared (UBJSON)	7m 21s
windows-msvc-shared (FLEXBUFFERS)	5m 29s
windows-msvc-shared (CBOR)	8m 46s
windows-msvc-shared (PARQUET)	39m 52s
windows-msvc-shared (MSGPACK)	4m 11s
windows-msvc-shared (CAPNPROTO)	7m 58s
windows-msvc (AVRO)	7m 41s
windows-msvc (TOML)	3m 41s
windows-msvc (CAPNPROTO)	8m 10s
windows-msvc (CBOR)	9m 33s
windows-msvc (UBJSON)	8m 22s
windows-msvc (PARQUET)	39m 10s
windows-msvc (JSON)	11m 4s
windows-msvc (XML)	4m 4s
windows-msvc (YAML)	4m 35s
windows-msvc (benchmarks)	46m 38s
windows-msvc (MSGPACK)	3m 40s
windows-msvc (FLEXBUFFERS)	6m 30s
	4h 16m 45s

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