fixup! Add word-wrap support, with wrap length provided by the user

document wrapAt in the HelpPrinterCustom comments

help.go

@@ -59,6 +59,11 @@ var HelpPrinter helpPrinter = printHelp
 // HelpPrinterCustom is a function that writes the help output. It is used as
 // the default implementation of HelpPrinter, and may be called directly if
 // the ExtraInfo field is set on an App.
+//
+// In the default implementation, if the customFuncs argument contains a
+// "wrapAt" key, which is a function which takes no arguments and returns
+// an int, this int value will be used to produce a "wrap" function used
+// by the default template to wrap long lines.
 var HelpPrinterCustom helpPrinterCustom = printHelpCustom
 
 // VersionPrinter prints the version for the App