Update docstrings for secret types (#8442)

pydantic/types.py

@@ -1535,7 +1535,8 @@ def _secret_display(value: str | bytes) -> str:
 class SecretStr(_SecretField[str]):
     """A string used for storing sensitive information that you do not want to be visible in logging or tracebacks.
 
-    It displays `'**********'` instead of the string value on `repr()` and `str()` calls.
+    When the secret value is nonempty, it is displayed as `'**********'` instead of the underlying value in
+    calls to `repr()` and `str()`. If the value _is_ empty, it is displayed as `''`.
 
     ```py
     from pydantic import BaseModel, SecretStr
@@ -1550,6 +1551,8 @@ class User(BaseModel):
     #> username='scolvin' password=SecretStr('**********')
     print(user.password.get_secret_value())
     #> password1
+    print((SecretStr('password'), SecretStr('')))
+    #> (SecretStr('**********'), SecretStr(''))
     ```
     """
 
@@ -1561,6 +1564,8 @@ class SecretBytes(_SecretField[bytes]):
     """A bytes used for storing sensitive information that you do not want to be visible in logging or tracebacks.
 
     It displays `b'**********'` instead of the string value on `repr()` and `str()` calls.
+    When the secret value is nonempty, it is displayed as `b'**********'` instead of the underlying value in
+    calls to `repr()` and `str()`. If the value _is_ empty, it is displayed as `b''`.
 
     ```py
     from pydantic import BaseModel, SecretBytes
@@ -1573,6 +1578,8 @@ class User(BaseModel):
     #> username='scolvin' password=SecretBytes(b'**********')
     print(user.password.get_secret_value())
     #> b'password1'
+    print((SecretBytes(b'password'), SecretBytes(b'')))
+    #> (SecretBytes(b'**********'), SecretBytes(b''))
     ```
     """