Clean javadoc tags and strip html.
Methods and constructors have an optional `javadoc` field.  All fields under
`javadoc` are optional but at least one will be present.

Fields also have optional `javadoc` field which, if present, is a string.
```
"javadoc": {
  "description": "...",

  // from @param <param name> <param description>
  "parameters": {
    "p1": "<p1 description>",
    "p2": "<p2 description>"
  },

  // from @return
  "return": "...",

  // from @throws <type> <description>
  "throws": [
    [
      "IndexOutOfBoundsException",
      "<description>"
    ],
    [
      "IOException",
      "<description>"
    ]
  ]
}
```

…ing/jdk-src-parse-05-clean-at