gopls/internal/lsp/frob: document another restriction

Change-Id: Iaaa21c431f20ebd708713884583cfbe59a82d64b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/511716
Run-TryBot: Alan Donovan <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
gopls-CI: kokoro <[email protected]>

Author: adonovan

gopls/internal/lsp/frob/frob.go

@@ -19,6 +19,9 @@
 //
 //   - Types that (recursively) contain private struct fields are not permitted.
 //
+//   - Values are serialized as trees, not graphs, so shared subgraphs
+//     are encoded repeatedly.
+//
 //   - No attempt is made to detect cyclic data structures.
 package frob