improve api doc readability

before: ![8byd8ucrtzrak1-dvssxjr76rnilmk8vmv8l0ittm_4](https://cloud.githubusercontent.com/assets/1377/3635081/38ab3e92-0f5b-11e4-8f87-a63c03d49d69.png)

after: ![foxdzz6obslzqhzr0nnd_ruhzniyqbkdm5beywuxdrq](https://cloud.githubusercontent.com/assets/1377/3635083/3c96ae42-0f5b-11e4-88cb-57936baa0430.png)