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/pm.go

@@ -68,7 +68,7 @@ type PolicyManager interface {
 	DebugString() string
 }
 
-// NewPolicyManager returns a new policy manager.
+// NewPolicyManager returns a new [PolicyManager].
 func NewPolicyManager(pol []byte, users []types.User, nodes views.Slice[types.NodeView]) (PolicyManager, error) {
 	var (
 		polMan PolicyManager
@@ -83,8 +83,8 @@ func NewPolicyManager(pol []byte, users []types.User, nodes views.Slice[types.No
 	return polMan, err
 }
 
-// PolicyManagersForTest returns all available PostureManagers to be used
-// in tests to validate them in tests that try to determine that they
+// PolicyManagersForTest returns all available [PolicyManager] implementations to
+// be used in tests to validate them in tests that try to determine that they
 // behave the same.
 func PolicyManagersForTest(pol []byte, users []types.User, nodes views.Slice[types.NodeView]) ([]PolicyManager, error) {
 	var polMans []PolicyManager