Ignore 'docs/node_modules'

`yuidoc` is installed into this directory.
Avoid incorrect tag warning

```
warn: (docparser): replacing incorrect tag: returns with return:
```

I rewrite `returns` -> `return`.
Merge pull request #985 from tricknotes/avoid-incorrect-tag-warning

Avoid incorrect tag warning
Merge pull request #984 from tricknotes/ignore-node-modules

Ignore 'docs/node_modules'