Add warning message to `session save` when transcript isn't saved. (#109020)

Somewhat recently, we made the change to hide the behavior to save LLDB
session history to the transcript buffer behind the flag
`interpreter.save-transcript`. By default, `interpreter.save-transcript`
is false. See #90703 for context.

I'm making a small update here to our `session save` messaging and some
help docs to clarify for users that aren't aware of this change. Maybe
`interpreter.save-transcript` could be true by default as well. Any
feedback welcome.

# Tests
```
bin/lldb-dotest -p TestSessionSave
```

---------

Co-authored-by: Tom Yang <toyang@fb.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/lldb/source/Interpreter/CommandInterpreter.cpp b/lldb/source/Interpreter/CommandInterpreter.cpp
index d17aa6f..8d3a82e 100644
--- a/lldb/source/Interpreter/CommandInterpreter.cpp
+++ b/lldb/source/Interpreter/CommandInterpreter.cpp
@@ -3308,6 +3308,10 @@ bool CommandInterpreter::SaveTranscript(
   result.SetStatus(eReturnStatusSuccessFinishNoResult);
   result.AppendMessageWithFormat("Session's transcripts saved to %s\n",
                                  output_file->c_str());
+  if (!GetSaveTranscript())
+    result.AppendError(
+        "Note: the setting interpreter.save-transcript is set to false, so the "
+        "transcript might not have been recorded.");
 
   if (GetOpenTranscriptInEditor() && Host::IsInteractiveGraphicSession()) {
     const FileSpec file_spec;