partners[openai]: update the docstring of OpenAI, OpenAIEmbeddings and ChatOpenAI classes (#18908)

**Description:** Update the docstring of OpenAI, OpenAIEmbeddings and
ChatOpenAI classes
**Issue:** Update import module paths to the current LangChain API
**Dependencies:** None
**Lint and test**: `make format` and `make lint` were run

This incorporates the review comments from langchain-ai/langchain#18637
which I closed due to an issue I had in updating that pr branch

---------

Co-authored-by: Erick Friis <erick@langchain.dev>

libs/partners/openai/langchain_openai/chat_models/base.py

@@ -225,8 +225,8 @@ class _AllReturnType(TypedDict):
 class ChatOpenAI(BaseChatModel):
     """`OpenAI` Chat large language models API.
 
-    To use, you should have the
-    environment variable ``OPENAI_API_KEY`` set with your API key.
+    To use, you should have the environment variable ``OPENAI_API_KEY``
+    set with your API key, or pass it as a named parameter to the constructor.
 
     Any parameters that are valid to be passed to the openai.create call can be passed
     in, even if not explicitly saved on this class.
@@ -234,8 +234,9 @@ class ChatOpenAI(BaseChatModel):
     Example:
         .. code-block:: python
 
-            from langchain_community.chat_models import ChatOpenAI
-            openai = ChatOpenAI(model_name="gpt-3.5-turbo")
+            from langchain_openai import ChatOpenAI
+
+            model = ChatOpenAI(model_name="gpt-3.5-turbo")
     """
 
     @property

libs/partners/openai/langchain_openai/embeddings/base.py

@@ -44,16 +44,15 @@ class OpenAIEmbeddings(BaseModel, Embeddings):
     environment variable ``OPENAI_API_KEY`` set with your API key or pass it
     as a named parameter to the constructor.
 
+    In order to use the library with Microsoft Azure endpoints, use
+    AzureOpenAIEmbeddings.
+
     Example:
         .. code-block:: python
 
             from langchain_openai import OpenAIEmbeddings
 
-            openai = OpenAIEmbeddings(model="text-embedding-3-large")
-
-    In order to use the library with Microsoft Azure endpoints, use
-        AzureOpenAIEmbeddings.
-
+            model = OpenAIEmbeddings(model="text-embedding-3-large")
     """
 
     client: Any = Field(default=None, exclude=True)  #: :meta private:

libs/partners/openai/langchain_openai/llms/base.py

@@ -603,8 +603,8 @@ class BaseOpenAI(BaseLLM):
 class OpenAI(BaseOpenAI):
     """OpenAI large language models.
 
-    To use, you should have the ``openai`` python package installed, and the
-    environment variable ``OPENAI_API_KEY`` set with your API key.
+    To use, you should have the environment variable ``OPENAI_API_KEY``
+    set with your API key, or pass it as a named parameter to the constructor.
 
     Any parameters that are valid to be passed to the openai.create call can be passed
     in, even if not explicitly saved on this class.
@@ -612,8 +612,9 @@ class OpenAI(BaseOpenAI):
     Example:
         .. code-block:: python
 
-            from langchain_community.llms import OpenAI
-            openai = OpenAI(model_name="gpt-3.5-turbo-instruct")
+            from langchain_openai import OpenAI
+
+            model = OpenAI(model_name="gpt-3.5-turbo-instruct")
     """
 
     @classmethod