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/dns/extrarecords.go

@@ -30,7 +30,7 @@ type ExtraRecordsMan struct {
 	hashes   map[string][32]byte
 }
 
-// NewExtraRecordsManager creates a new ExtraRecordsMan and starts watching the file at the given path.
+// NewExtraRecordsManager creates a new [ExtraRecordsMan] and starts watching the file at the given path.
 func NewExtraRecordsManager(path string) (*ExtraRecordsMan, error) {
 	watcher, err := fsnotify.NewWatcher()
 	if err != nil {
@@ -177,7 +177,7 @@ func (e *ExtraRecordsMan) updateRecords() {
 	e.updateCh <- e.records.Slice()
 }
 
-// readExtraRecordsFromPath reads a JSON file of tailcfg.DNSRecord
+// readExtraRecordsFromPath reads a JSON file of [tailcfg.DNSRecord]
 // and returns the records and the hash of the file.
 func readExtraRecordsFromPath(path string) ([]tailcfg.DNSRecord, [32]byte, error) {
 	b, err := os.ReadFile(path)