docs: Update func doc strings in tools_human (#26149)

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- **Description:** Fix docstring for two functions that look like have
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---------

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

docs/docs/how_to/tools_human.ipynb

@@ -129,13 +129,13 @@
     "\n",
     "@tool\n",
     "def count_emails(last_n_days: int) -> int:\n",
-    "    \"\"\"Multiply two integers together.\"\"\"\n",
+    "    \"\"\"Dummy function to count number of e-mails. Returns 2 * last_n_days.\"\"\"\n",
     "    return last_n_days * 2\n",
     "\n",
     "\n",
     "@tool\n",
     "def send_email(message: str, recipient: str) -> str:\n",
-    "    \"Add two integers.\"\n",
+    "    \"\"\"Dummy function for sending an e-mail.\"\"\"\n",
     "    return f\"Successfully sent email to {recipient}.\"\n",
     "\n",
     "\n",