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/handlers.go

@@ -129,7 +129,7 @@ func parseCapabilityVersion(req *http.Request) (tailcfg.CapabilityVersion, error
 }
 
 // verifyBodyLimit caps the request body for /verify. The DERP verify
-// protocol payload (tailcfg.DERPAdmitClientRequest) is a few hundred
+// protocol payload ([tailcfg.DERPAdmitClientRequest]) is a few hundred
 // bytes; 4 KiB is generous and prevents an unauthenticated client from
 // OOMing the public router with arbitrarily large POSTs.
 const verifyBodyLimit int64 = 4 * 1024
@@ -358,7 +358,7 @@ func authIDFromRequest(req *http.Request) (types.AuthID, error) {
 // Listens in /register/:registration_id.
 //
 // This is not part of the Tailscale control API, as we could send whatever URL
-// in the RegisterResponse.AuthURL field.
+// in the [tailcfg.RegisterResponse.AuthURL] field.
 func (a *AuthProviderWeb) RegisterHandler(
 	writer http.ResponseWriter,
 	req *http.Request,