flag: add comment stating that Set is called in sequence across the command line

No code changes.

Change-Id: I3b78b1048318a4b80747fde8cab919282fc444a8
Reviewed-on: https://go-review.googlesource.com/12285Reviewed-by: Andrew Gerrand <adg@golang.org>

src/flag/flag.go

@@ -235,6 +235,8 @@ func (d *durationValue) String() string { return (*time.Duration)(d).String() }
 // If a Value has an IsBoolFlag() bool method returning true,
 // the command-line parser makes -name equivalent to -name=true
 // rather than using the next command-line argument.
+//
+// Set is called once, in command line order, for each flag present.
 type Value interface {
 	String() string
 	Set(string) error