Refactor API docs

Refer to `vfile` for more up-to-date docs.

Related: why-jay/remark-lint-sentence-newline#2

doc/api.md

@@ -21,7 +21,7 @@ var doc = '* Hello\n' +
 Process.
 
 ```javascript
-processor.process(doc, function (err, res, file) {
+processor.process(doc, function (err, file, res) {
     var messages = file.messages;
     /**
      * Yields:
@@ -68,37 +68,5 @@ look as follows:
 }
 ```
 
-These messages comply to two schema’s. First, they’re valid error objects,
-so they can be thrown (while still looking :ok_hand:, due to `Error#toString()`
-magic), secondly the `file` object itself can be passed to ESLint’s formatters:
-
-```javascript
-var compact = require('eslint/lib/formatters/compact');
-var mdast = require('mdast');
-var lint = require('mdast-lint');
-
-// For example purposes were processing `example.md` as seen above.
-mdast().use(lint).process(doc, function (err, res, file) {
-    compact([file]);
-    /**
-     * line 3, col 1, Warning - Invalid ordered list item marker: should be `*`
-     *
-     * 1 problem
-     */
-});
-```
-
-Each message contains the following properties:
-
-*   `fatal` (`boolean?`) — Always `false`;
-
-*   `reason` (`string`) — Warning message;
-
-*   `line` (`number`) — Starting line (1-based);
-
-*   `column` (`number`) — Starting column of exception (1-based).
-
-*   `file` (`string`) — File path of exception.
-
-*   `ruleId` (`string`) — Name of the rule which triggered this warning,
-    useful when looking for how to turn the darn thing off.
+See [`VFileMessage`](https://github.com/wooorm/vfile#vfilemessage) for more
+information.