fix usage message for func flags, fix arguments order

[LeGEC]

• fix usage message for BoolFunc and Func flags Improve help text for a flag added by BoolFunc #430
• fix arguments order fix order of arguments in new functions .Func() and .BoolFunc() #433

[LeGEC]

I'm not sure whether I updated the correct locations in code to change the generated usage message for "func" and "boolfunc" flags, feel free to comment (or to overwrite the code)

[tomasaschan]

I think this seems to reproduce the stdlib behavior close enough:

❯ go run ./main.go -h # version using stdlib flag
Usage of /tmp/go-build4240827856/b001/exe/main:
  -b    bool func flag
  -f value
        func flag

❯ go run ./main.go -h # version using pflag on this PR branch
Usage of /tmp/go-build1174874912/b001/exe/main:
      --b         bool func flag
      --f value   func flag
pflag: help requested
exit status 2

The last two lines will be addressed by #306 (I think) and the line break difference is probably not just this flag.

Code for the above output is

package main

import (
	"flag"

	"github.com/spf13/pflag"
)

func stdlib_flag() {
	flag.Func("f", "func flag", func(s string) error {
		println("flag f called with", s)
		return nil
	})

	flag.BoolFunc("b", "bool func flag", func(b string) error {
		println("flag b called with", b)
		return nil
	})

	flag.Parse()

	println("flag f value:", flag.Lookup("f").Value.String())
	println("flag b value:", flag.Lookup("b").Value.String())
}

func p_flag() {
	pflag.Func("f", func(s string) error {
		println("flag f called with", s)
		return nil
	}, "func flag")

	pflag.BoolFunc("b", "bool func flag", func(b string) error {
		println("flag b called with", b)
		return nil
	})

	pflag.Parse()

	println("flag f value:", pflag.Lookup("f").Value.String())
	println("flag b value:", pflag.Lookup("b").Value.String())
}

func main() {
	p_flag() # or stdlib_flag() depending on what we want to look at
}

Note the unfortunate argument order difference for Func; maybe we should fix that (separately)?

[LeGEC]

created the issue for arguments order, added a separate commit to fix it

I think it is ok to put both fixes in a single Pull Request ?