Fix an obsolete comment

The old comment was probably copypasted from CmdBuild. Now it reflects
what new-run actually does.

Author: fgaz

cabal-install/Distribution/Client/CmdRun.hs

@@ -100,9 +100,10 @@ runCommand = Client.installCommand {
    }
 
 
--- | The @build@ command does a lot. It brings the install plan up to date,
--- selects that part of the plan needed by the given or implicit targets and
--- then executes the plan.
+-- | The @run@ command runs a specified executable-like component, building it
+-- first if necessary. The component can be either an executable, a test,
+-- or a benchmark. This is particularly useful for passing arguments to
+-- exes/tests/benchs by simply appending them after a @--@.
 --
 -- For more details on how this works, see the module
 -- "Distribution.Client.ProjectOrchestration"