all: apply godoc [Name] link conventions across comments

Every Go-identifier reference in // and /* */ comments now uses
godoc's [Name] linking syntax so pkg.go.dev and `go doc` render
them as clickable cross-references. No behaviour change.

Pattern applied across the tree:
  In-package         [Foo], [Foo.Bar]
  Cross-package      [pkg.Foo], [pkg.Foo.Bar]
  Stdlib             [netip.Prefix], [errors.Is], [context.Context]
  Tailscale          [tailcfg.MapResponse], [tailcfg.Node.CapMap],
                     [tailcfg.NodeAttrSuggestExitNode]

Skip rules:
  - File:line refs left as plain text
  - HuJSON wire keys inside backtick raw strings untouched
  - ACL/policy syntax tokens (tag:foo, autogroup:self, ...) not Go
    symbols, left as plain text
  - JSON/OIDC wire keys, gorm tags, RFC IPv6 placeholders, markdown
    link tags, decorative dividers — all left as-is

hscontrol/util/util.go

@@ -94,7 +94,7 @@ type Traceroute struct {
 	Err error
 }
 
-// ParseTraceroute parses the output of the traceroute command and returns a Traceroute struct.
+// ParseTraceroute parses the output of the traceroute command and returns a [Traceroute] struct.
 func ParseTraceroute(output string) (Traceroute, error) {
 	lines := strings.Split(strings.TrimSpace(output), "\n")
 	if len(lines) < 1 {