Document how to use format_str()

Closes #1064

black.py

@@ -746,7 +746,30 @@ def format_str(src_contents: str, *, mode: FileMode) -> FileContent:
     """Reformat a string and return new contents.
 
     `mode` determines formatting options, such as how many characters per line are
-    allowed.
+    allowed.  Example:
+
+    >>> import black
+    >>> print(black.format_str("def f(arg:str='')->None:...", mode=FileMode()))
+    def f(arg: str = "") -> None:
+        ...
+
+    A more complex example:
+    >>> print(
+    ...   black.format_str(
+    ...     "def f(arg:str='')->None: hey",
+    ...     mode=black.FileMode(
+    ...       target_versions={black.TargetVersion.PY36},
+    ...       line_length=10,
+    ...       string_normalization=False,
+    ...       is_pyi=False,
+    ...     ),
+    ...   ),
+    ... )
+    def f(
+        arg: str = '',
+    ) -> None:
+        hey
+
     """
     src_node = lib2to3_parse(src_contents.lstrip(), mode.target_versions)
     dst_contents = []