cmd/test2json: document missing "skip" action
cmd/test2json: document missing "skip" action

Change-Id: I906e61170279f0647598e2fd4fa931aac1b69288
GitHub-Last-Rev: f6df43e8e10e3b032a67490611c0ba5ad8e948df
GitHub-Pull-Request: golang/go#24517
Reviewed-on: https://go-review.googlesource.com/102396
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

CAFxX/go,Cofyc/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,josharian/go,CAFxX/go,cloudflare/go,Cofyc/go,nawawi/go,cloudflare/go,nawawi/go,josharian/go,cloudflare/go,Cofyc/go,nawawi/go,cloudflare/go,golang/go,josharian/go,akutz/go,josharian/go,Cofyc/go,akutz/go,nawawi/go,akutz/go,akutz/go,CAFxX/go,golang/go,CAFxX/go,golang/go,josharian/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,Cofyc/go,akutz/go,CAFxX/go,Cofyc/go,cloudflare/go,josharian/go,golang/go,Cofyc/go,golang/go,josharian/go,josharian/go,cloudflare/go,akutz/go,akutz/go,nawawi/go,akutz/go,golang/go,Cofyc/go