docs: Add missing docstrings, adjust wording to match standard style

This patch adds a missing docstrings for exported identifiers, and
adjust some of the existing ones to match the standard style.

In some cases, the identifiers were un-exported after noticing they had
no external users.

Besides improving documentation, it also reduces the linter noise
significantly.

internal/set/set.go

@@ -1,16 +1,19 @@
 // Package set implement sets for various types. Well, only string for now :)
 package set
 
+// String set.
 type String struct {
 	m map[string]struct{}
 }
 
+// NewString returns a new string set, with the given values in it.
 func NewString(values ...string) *String {
 	s := &String{}
 	s.Add(values...)
 	return s
 }
 
+// Add values to the string set.
 func (s *String) Add(values ...string) {
 	if s.m == nil {
 		s.m = map[string]struct{}{}
@@ -21,6 +24,7 @@ func (s *String) Add(values ...string) {
 	}
 }
 
+// Has checks if the set has the given value.
 func (s *String) Has(value string) bool {
 	// We explicitly allow s to be nil *in this function* to simplify callers'
 	// code.  Note that Add will not tolerate it, and will panic.