Add documentation for bytes formatting error code (#14971)

See #14959

Author: hauntsaninja

Diff for: docs/source/error_code_list.rst

@@ -908,6 +908,24 @@ Functions will always evaluate to true in boolean contexts.
     if f:  # Error: Function "Callable[[], Any]" could always be true in boolean context  [truthy-function]
         pass
 
+Check for implicit bytes coercions [str-bytes-safe]
+-------------------------------------------------------------------
+
+Warn about cases where a bytes object may be converted to a string in an unexpected manner.
+
+.. code-block:: python
+
+    b = b"abc"
+
+    # Error: If x = b'abc' then f"{x}" or "{}".format(x) produces "b'abc'", not "abc".
+    # If this is desired behavior, use f"{x!r}" or "{!r}".format(x).
+    # Otherwise, decode the bytes [str-bytes-safe]
+    print(f"The alphabet starts with {b}")
+
+    # Okay
+    print(f"The alphabet starts with {b!r}")  # The alphabet starts with b'abc'
+    print(f"The alphabet starts with {b.decode('utf-8')}")  # The alphabet starts with abc
+
 Report syntax errors [syntax]
 -----------------------------
 

Diff for: mypy/errorcodes.py

@@ -113,7 +113,7 @@ def __str__(self) -> str:
     "str-format", "Check that string formatting/interpolation is type-safe", "General"
 )
 STR_BYTES_PY3: Final = ErrorCode(
-    "str-bytes-safe", "Warn about dangerous coercions related to bytes and string types", "General"
+    "str-bytes-safe", "Warn about implicit coercions related to bytes and string types", "General"
 )
 EXIT_RETURN: Final = ErrorCode(
     "exit-return", "Warn about too general return type for '__exit__'", "General"