cmd/doc: usage()

[gertcuykens]

Fixes usage inconsistency between

/usr/local/go/src/cmd/doc/main.go

func usage() {
	fmt.Fprintf(os.Stderr, "Usage of [go] doc:\n")
	fmt.Fprintf(os.Stderr, "\tgo doc\n")
	fmt.Fprintf(os.Stderr, "\tgo doc <pkg>\n")
	fmt.Fprintf(os.Stderr, "\tgo doc <pkg> <sym>[.<method>]\n")
	fmt.Fprintf(os.Stderr, "\tgo doc [<pkg>.][<sym>.]<method>\n")
	fmt.Fprintf(os.Stderr, "\tgo doc <sym>[.<method>]\n")
	fmt.Fprintf(os.Stderr, "For more information run\n")
	fmt.Fprintf(os.Stderr, "\tgo help doc\n\n")
	fmt.Fprintf(os.Stderr, "Flags:\n")
	flag.PrintDefaults()
	os.Exit(2)
}

/usr/local/go/src/cmd/go/internal/doc/doc.go

	go doc <pkg>
	go doc <pkg> <sym>[.<methodOrField>]
	go doc [<pkg>.][<sym>.]<methodOrField>
	go doc <sym>[.<methodOrField>]

see diff https://golang.org/cl/201857 for details
Note that the original [<pkg>].<sym>[.<method>] was definitely a typo ]. vs .] ,just trying to make it identical while i was at it fixing that.

Also note https://groups.google.com/forum/#!topic/golang-nuts/OFg-02kfpno Compared to go help doc output, it seems flag.PrintDefaults() messes up flags descriptions new lines of go doc --help? But thats probably a separate issue with flag.PrintDefaults()

[gopherbot]

Change https://golang.org/cl/201857 mentions this issue: cmd/doc: usage()

[julieqiu]

/cc @jayconrod @bcmills

[gertcuykens]

@julieqiu please add label Go 1.14 and remove needsInvestigation before this patch gets forgotten

[bcmills]

@gertcuykens, as far as I can tell this fix doesn't need to hold up the 1.14 release if it isn't ready, so the Go1.14 milestone probably isn't appropriate. (But it's small and cosmetic enough that it can probably make 1.14 anyway.)