docs: change for optional variables in chatprompt (#25017)

Fixes #24884

libs/core/langchain_core/prompts/base.py

@@ -46,7 +46,9 @@ class BasePromptTemplate(
     """A list of the names of the variables whose values are required as inputs to the 
     prompt."""
     optional_variables: List[str] = Field(default=[])
-    """A list of the names of the variables that are optional in the prompt."""
+    """optional_variables: A list of the names of the variables for placeholder
+       or MessagePlaceholder that are optional. These variables are auto inferred 
+       from the prompt and user need not provide them."""
     input_types: Dict[str, Any] = Field(default_factory=dict, exclude=True)
     """A dictionary of the types of the variables the prompt template expects.
     If not provided, all variables are assumed to be strings."""

libs/core/langchain_core/prompts/chat.py

@@ -946,8 +946,9 @@ class ChatPromptTemplate(BaseChatPromptTemplate):
             template_format: format of the template. Defaults to "f-string".
             input_variables: A list of the names of the variables whose values are
                 required as inputs to the prompt.
-            optional_variables: A list of the names of the variables that are optional
-                in the prompt.
+            optional_variables: A list of the names of the variables for placeholder
+            or MessagePlaceholder that are optional. These variables are auto inferred
+            from the prompt and user need not provide them.
             partial_variables: A dictionary of the partial variables the prompt
                 template carries. Partial variables populate the template so that you
                 don't need to pass them in every time you call the prompt.