[Documentation] Show how to initiate pinecone from an existing index (#3070)

## What is this PR for:
* This PR adds a commented line of code in the documentation that shows
how someone can use the Pinecone client with an already existing
Pinecone index
* The documentation currently only shows how to create a pinecone index
from langchain documents but not how to load one that already exists

docs/modules/indexes/vectorstores/examples/pinecone.ipynb

@@ -58,6 +58,9 @@
     "\n",
     "docsearch = Pinecone.from_documents(docs, embeddings, index_name=index_name)\n",
     "\n",
+    "# if you already have an index, you can load it like this\n",
+    "# docsearch = Pinecone.from_existing_index(index_name, embeddings)\n",
+    "\n",
     "query = \"What did the president say about Ketanji Brown Jackson\"\n",
     "docs = docsearch.similarity_search(query)"
    ]