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/policy/v2/utils.go

@@ -31,8 +31,8 @@ var (
 //
 // Brackets are only accepted around IPv6 addresses, not IPv4, hostnames, or other alias types.
 // Bracket stripping reduces both forms to bare "addr:port" or "addr/prefix:port",
-// which the normal LastIndex(":") split handles correctly because port strings
-// never contain colons.
+// which the normal [strings.LastIndex] of ":" split handles correctly because
+// port strings never contain colons.
 func splitDestinationAndPort(input string) (string, string, error) {
 	// Handle RFC 3986 bracketed IPv6 (e.g. "[::1]:80" or "[fd7a::1]/128:80,443").
 	// Strip brackets after validation and fall through to normal parsing.
@@ -82,7 +82,7 @@ func splitDestinationAndPort(input string) (string, string, error) {
 	return destination, port, nil
 }
 
-// parsePortRange parses a port definition string and returns a slice of PortRange structs.
+// parsePortRange parses a port definition string and returns a slice of [tailcfg.PortRange] structs.
 func parsePortRange(portDef string) ([]tailcfg.PortRange, error) {
 	if portDef == "*" {
 		return []tailcfg.PortRange{tailcfg.PortRangeAny}, nil