docs(backups): add backup_id provenance hint in create_index_from_backup

## Purpose

Users calling `create_index_from_backup` have no indication of how to
obtain a valid `backup_id`, which is a required parameter with no
obvious default. Missing navigation hint violates the convention that
second-step methods must reference their prerequisites.

## Solution

Added a one-sentence prose reference to the `backup_id` parameter
description in both the sync (`Pinecone`) and async (`AsyncPinecone`)
clients pointing to `backups.create` and `backups.list` as the two
canonical sources for a backup identifier. Chose prose over inline
workflow example to keep the docstring focused on this method.

Author: jhamon

pinecone/_client.py

@@ -474,7 +474,8 @@ def create_index_from_backup(
 
         Args:
             name (str): Name for the new index.
-            backup_id (str): Identifier of the backup to restore from.
+            backup_id (str): Identifier of the backup to restore from.  Obtain this
+                from :meth:`Pinecone.backups.create` or :meth:`Pinecone.backups.list`.
             deletion_protection (DeletionProtection | str | None): ``"enabled"`` or
                 ``"disabled"``. Defaults to ``"disabled"`` server-side when omitted.
             tags (Mapping[str, str] | None): Optional key-value tags for the new index.

pinecone/async_client/pinecone.py

@@ -368,7 +368,8 @@ async def create_index_from_backup(
 
         Args:
             name (str): Name for the new index.
-            backup_id (str): Identifier of the backup to restore from.
+            backup_id (str): Identifier of the backup to restore from.  Obtain this
+                from :meth:`AsyncPinecone.backups.create` or :meth:`AsyncPinecone.backups.list`.
             deletion_protection (DeletionProtection | str | None): ``"enabled"`` or
                 ``"disabled"``. Defaults to ``"disabled"`` server-side when omitted.
             tags (Mapping[str, str] | None): Optional key-value tags for the new index.