**kwargs usage breaks generation for macros

[thomasvl]

Given:

def my_macro(name, a, b=[], c="blah", **kwargs):
    """Mumble

    Args:
      name: The name
      a: An a
      b: List of b
      c: A string
      **kwargs: The <a href="https://docs.bazel.build/versions/master/be/common-definitions.html#common-attributes-tests">common attributes for tests</a>.

    """
    pass

You get generated markdown of:

## my_macro

<pre>
my_macro(<a href="#my_macro-name">name</a>, <a href="#my_macro-a">a</a>, <a href="#my_macro-b">b</a>, <a href="#my_macro-c">c</a>, <a href="#my_macro-kwargs">kwargs</a>)
</pre>

Mumble

### Parameters

<table class="params-table">
  <colgroup>
    <col class="col-param" />
    <col class="col-description" />
  </colgroup>
  <tbody>
    <tr id="my_macro-name">
      <td><code>name</code></td>
      <td>
        required.
        <p>
          The name
        </p>
      </td>
    </tr>
    <tr id="my_macro-a">
      <td><code>a</code></td>
      <td>
        required.
        <p>
          An a
        </p>
      </td>
    </tr>
    <tr id="my_macro-b">
      <td><code>b</code></td>
      <td>
        required.
        <p>
          List of b
        </p>
      </td>
    </tr>
    <tr id="my_macro-c">
      <td><code>c</code></td>
      <td>
        optional. default is <code>[]</code>
        <p>
          A string
        </p>
      </td>
    </tr>
    <tr id="my_macro-kwargs">
      <td><code>kwargs</code></td>
      <td>
        optional. default is <code>"blah"</code>
      </td>
    </tr>
  </tbody>
</table>

Which renders to:

---

my_macro

my_macro(name, a, b, c, kwargs)

Mumble

Parameters

name	required. The name
a	required. An a
b	required. List of b
c	optional. default is [] A string
kwargs	optional. default is "blah"

---

What's worth noting here:

1. b, c, and kwargs have shifted default values.
2. b is listed as required, but it has a default, like to do with the previous issue of displaying wrong defaults.
3. kwargs is missing its description.

If you change the **kwargs in the Args section to just kwargs, then description is displayed, but the other issues still apply.