doc: make Cli.Cmd.validate doc more precise

Author: mhuisi

Diff for: Cli/Basic.lean

@@ -1333,7 +1333,7 @@ section IO
     returns `0` for the exit code.
     If neither of these flags were passed and processing was successful, the `run` handler of the
     called command is executed.
-    In the case of a processing error, the error is printed and an exit code of `1` is returned.
+    In the case of a processing error, the error is printed to stderr and an exit code of `1` is returned.
     -/
     def validate (c : Cmd) (args : List String) : IO UInt32 := do
       let result := c.process args

Diff for: README.md

@@ -194,7 +194,7 @@ Prints the help or the version of the called (sub)command if the respective flag
 returns `0` for the exit code.
 If neither of these flags were passed and processing was successful, the `run` handler of the
 called command is executed.
-In the case of a processing error, the error is printed and an exit code of `1` is returned.
+In the case of a processing error, the error is printed to stderr and an exit code of `1` is returned.
 -/
 def validate (c : Cmd) (args : List String) : IO UInt32 := do
   let result := c.process args