doc: Update behaviour rationale

doc/rationale.md

@@ -24,6 +24,22 @@ The name Pantograph is a pun. It means two things
   a locomotive. In comparison the (relatively) simple Pantograph software powers
   theorem proving projects.
 
+## Caveats
+
+Pantograph does not exactly mimic Lean LSP's behaviour. That would not grant the
+flexibility it offers.  To support tree search means Pantograph has to act
+differently from Lean in some times, but never at the sacrifice of soundness.
+
+- When Lean LSP says "don't know how to synthesize placeholder", this indicates
+  the human operator needs to manually move the cursor to the placeholder and
+  type in the correct expression. This error therefore should not halt the proof
+  process, and the placeholder should be turned into a goal.
+- When Lean LSP says "unresolved goals", that means a proof cannot finish where
+  it is supposed to finish at the end of a `by` block. Pantograph will raise the
+  error in this case, since it indicates the termination of a proof search branch.
+- `pick_goal` or `swap` will not work since they run contrary to tree search
+  paradigms.
+
 ## References
 
 * [Pantograph Paper](https://arxiv.org/abs/2410.16429)