✨ Support Python internal description on Pydantic model's docstring
diff --git a/fastapi/utils.py b/fastapi/utils.py
index 0ced0125223a4..89f54453b5a90 100644
--- a/fastapi/utils.py
+++ b/fastapi/utils.py
@@ -37,6 +37,8 @@ def get_model_definitions(
         )
         definitions.update(m_definitions)
         model_name = model_name_map[model]
+        if "description" in m_schema:
+            m_schema["description"] = m_schema["description"].split("\f")[0]
         definitions[model_name] = m_schema
     return definitions
 

## Problem

Currently, on FastAPI endpoint functions we can document for OpenAPI description/Swagger and for internal Python purposes:
```python
from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

@app.get("/")
def useful():
    """My description.
    \f
    Internal documentation.
    """
    ...
```
On Swagger we'll have:
![image](https://user-images.githubusercontent.com/7353520/113333626-f3008200-9322-11eb-9f9c-ca6ba5782d25.png)
 
When documenting models, it doesn't have the same behavior:
```python
from enum import Enum
from fastapi import FastAPI

class Banana(str, Enum):
    """My description.
    \f
    Internal documentation.
    """
    HAHA = "haha"
    HOHO = "hoho"

app = FastAPI()

@app.get("/")
def useful(banana: Banana):
    ...
```
On Swagger, we'll have:
![image](https://user-images.githubusercontent.com/7353520/113333882-48d52a00-9323-11eb-8d0e-eef8cc965d04.png)
    
## Solution

The proposed _two lines of code_ on this PR, makes it possible to have the same behavior as the endpoint description:
![image](https://user-images.githubusercontent.com/7353520/113334003-6e623380-9323-11eb-849d-750a85f7847c.png)

It follows the same idea as: https://github.com/tiangolo/fastapi/blob/10397ddc30ef9bb26e9247f04fe20ce3045081c4/fastapi/routing.py#L350

## Related issues
- https://github.com/tiangolo/fastapi/issues/3024

## Questions & Alternatives
- Should Pydantic handle this on the [model_process_schema()](https://github.com/samuelcolvin/pydantic/blob/e68c277afe6bf6e7966bfecfe9b228d33f2de44b/pydantic/schema.py#L519) function instead?