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/poll_test.go

@@ -100,7 +100,7 @@ func (w *delayedSuccessResponseWriter) WriteCount() int {
 //  3. While that write is blocked, queue enough updates to fill the buffered
 //     channel and make the next batcher send hit the stale-send timeout.
 //  4. That stale-send path removes the session from the batcher, so without an
-//     explicit teardown hook the old serveLongPoll goroutine would stay alive
+//     explicit teardown hook the old [mapSession.serveLongPoll] goroutine would stay alive
 //     but stop receiving future updates.
 //  5. Release the blocked write and verify the batcher-side stop signal makes
 //     that stale session exit instead of lingering as an orphaned goroutine.