Keep a try off the forge and bound how many run (hq issue 686)

Review of #314: a try's start named its commit, which the forge's holder
matches to an open pull request's head and marks pending. Its start now
names no repository or commit. One try per commit and four in all, a try
never ages ahead of deliveries or pull request checks, and its id is
checked before it is a bus subject.

internal/catalogue/verbs.go

@@ -369,11 +369,14 @@ var ControllerVerbs = []Verb{
 		"once with an id; asked again with that id it waits up to 20 s and answers pass, fail or error, how long it " +
 		"took and the lines naming what failed — or that it is still waiting or running, to ask again. Sets no " +
 		"status on the forge, registers and builds nothing, runs no merge gate, counts as a check in the build queue " +
-		"(after deliveries) and is bounded as one. Only for a repository of the mesh; the ref must be on the forge.",
+		"(after deliveries and after every pull request's check, however long it waits) and is bounded as one. One " +
+		"try of a commit at a time — a second ask answers the first's id — and at most four waiting or running in " +
+		"all: a fifth is refused. Only for a repository of the mesh; the ref must be on the forge.",
 		Input: schema(map[string]string{
 			"repository": "the repository on the forge, owner/name; with ref",
 			"ref":        "the branch or commit to try, as pushed to the forge",
-			"base":       "the branch it is merged into (the repository's default branch when none is named)",
+			"base": "the branch it is merged into (the repository's default branch when none is named). The " +
+				"merge-check.sh that runs is that branch's, never the ref's own: the same trust as a pull request's base",
 			"tests": "only the tests this `go test -run` pattern names, where the repository's merge-check.sh reads " +
 				"MESH_CHECK_TESTS; a script that does not runs whole",
 			"store-only": "\"true\": only the repository's store tests, where its merge-check.sh reads " +