Merge BlockstreamResearch/simfony#95: Document jet cost #50

Job	Run time
Build - integration tests	1m 22s
Test - stable toolchain	55s
Test - MSRV toolchain	43s
Check flake	20s
Test - fuzzer (compile_parse_tree)	3m 57s
Test - fuzzer (compile_text)	3m 52s
Test - fuzzer (display_parse_tree)	3m 59s
Test - fuzzer (parse_value_rtt)	4m 1s
Test - fuzzer (parse_witness_json_rtt)	3m 56s
Test - fuzzer (parse_witness_module_rtt)	6m 19s
Test - fuzzer (reconstruct_value)	3m 56s
	33m 20s

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