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/servertest/assertions.go

@@ -96,7 +96,7 @@ func AssertPeerGone(tb testing.TB, observer *TestClient, peerName string) {
 }
 
 // AssertPeerHasAllowedIPs checks that a peer has the expected
-// AllowedIPs prefixes.
+// [tailcfg.Node.AllowedIPs] prefixes.
 func AssertPeerHasAllowedIPs(tb testing.TB, observer *TestClient, peerName string, want []netip.Prefix) {
 	tb.Helper()
 
@@ -211,7 +211,7 @@ func AssertSelfHasAddresses(tb testing.TB, client *TestClient) {
 	}
 }
 
-// EventuallyAssertMeshComplete retries AssertMeshComplete up to
+// EventuallyAssertMeshComplete retries [AssertMeshComplete] up to
 // timeout, useful when waiting for state to propagate.
 func EventuallyAssertMeshComplete(tb testing.TB, clients []*TestClient, timeout time.Duration) {
 	tb.Helper()