go/analysis/unitchecker: tell the user how to list the flags and analyzers

This is a lead-up to a change to create a better help messages in the go command.

Change-Id: I695fd2bf5420a62774334c77788e67565d3965c1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/291909
Trust: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
Reviewed-by: roger peppe <rogpeppe@gmail.com>

go/analysis/unitchecker/unitchecker.go

@@ -97,7 +97,7 @@ func Main(analyzers ...*analysis.Analyzer) {
 
 Usage of %[1]s:
 	%.16[1]s unit.cfg	# execute analysis specified by config file
-	%.16[1]s help    	# general help
+	%.16[1]s help    	# general help, including listing analyzers and flags
 	%.16[1]s help name	# help on specific analyzer and its flags
 `, progname)
 		os.Exit(1)