cmd/cue: clarify cue mod tidy --check help text

This tweaks the help text for "cue mod tidy" when invoked with the
"--check" flag, clarifying that (a) it still fetches and caches
dependencies and (b) it causes the command to fail if the module.cue
file would otherwise have been updated.

Signed-off-by: Jonathan Matthews <github@hello.jonathanmatthews.com>
Change-Id: If36a63346269c5abe2b2c6c585cd80898a224039
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1202607
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>

cmd/cue/cmd/modtidy.go

@@ -48,7 +48,7 @@ Note that this command is not yet stable and may be changed.
 		RunE: mkRunE(c, runModTidy),
 		Args: cobra.ExactArgs(0),
 	}
-	cmd.Flags().Bool(string(flagCheck), false, "check for tidiness only; do not update module.cue file")
+	cmd.Flags().Bool(string(flagCheck), false, "check for tidiness after fetching dependencies; fail if module.cue would be updated")
 
 	return cmd
 }