Add missing documentation to some coder classes #635

Job	Run time
Build	5s
Build	5s
Build	5s
Test (windows, head, 16.0-1-windows-x64, 16)	2m 23s
Test (windows, head, 16.0-1-windows-x64, 16)	2m 23s
Test (windows, head, 16.0-1-windows-x64, 16)	2m 23s
Test (windows, 2.5, 9.4.26-1-windows-x64, 9.4)	5m 45s
Test (windows, 2.5, 9.4.26-1-windows-x64, 9.4)	5m 45s
Test (windows, 2.5, 9.4.26-1-windows-x64, 9.4)	5m 45s
Test (windows, mswin, 16.0-1-windows-x64, 16)	2m 30s
Test (windows, mswin, 16.0-1-windows-x64, 16)	2m 30s
Test (windows, mswin, 16.0-1-windows-x64, 16)	2m 30s
Test (ubuntu, head, 16)	1m 1s
Test (ubuntu, head, 16)	1m 1s
Test (ubuntu, head, 16)	1m 1s
Test (ubuntu, 3.2, 12)	1m 9s
Test (ubuntu, 3.2, 12)	1m 9s
Test (ubuntu, 3.2, 12)	1m 9s
Test (ubuntu, 20.04, 2.5, 9.3)	1m 29s
Test (ubuntu, 20.04, 2.5, 9.3)	1m 29s
Test (ubuntu, 20.04, 2.5, 9.3)	1m 29s
Test (ubuntu, truffleruby, 13)	2m 4s
Test (ubuntu, truffleruby, 13)	2m 4s
Test (ubuntu, truffleruby, 13)	2m 4s
Test (ubuntu, truffleruby-head, 16)	1m 54s
Test (ubuntu, truffleruby-head, 16)	1m 46s
Test (ubuntu, truffleruby-head, 16)	1m 46s
Test (macos, head, 16.0-1-osx, 16)	2m 29s
Test (macos, head, 16.0-1-osx, 16)	2m 29s
Test (macos, head, 16.0-1-osx, 16)	1m 8s
	1h 0m 50s

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