doc: some function/vars/const/package level updates (#1558)

* doc: some function/vars/const/package level updates

Various update that stood out while reading godoc.org for CoreDNS.

* Fix some misspellings as well

plugin/pkg/fuzz/do.go

@@ -1,3 +1,4 @@
+// Package fuzz contains functions that enable fuzzing of plugins.
 package fuzz
 
 import (

plugin/pkg/parse/parse.go

@@ -1,3 +1,4 @@
+// Package parse contains functions that can be used in the setup code for plugins.
 package parse
 
 import (

plugin/pkg/rcode/rcode.go

@@ -6,8 +6,7 @@ import (
 	"github.com/miekg/dns"
 )
 
-// ToString convert the rcode to the official DNS string, or to "RCODE"+value if the RCODE
-// value is unknown.
+// ToString convert the rcode to the official DNS string, or to "RCODE"+value if the RCODE value is unknown.
 func ToString(rcode int) string {
 	if str, ok := dns.RcodeToString[rcode]; ok {
 		return str

plugin/pkg/up/up.go

@@ -1,3 +1,5 @@
+// Package up is used to run a function for some duration. If a new function is added while a previous run is
+// still ongoing, nothing new will be executed.
 package up
 
 import (