[Android] Format the description, add links and correct typos.

Continue to refine the description of embedding API:
1. Use '<p>' to format the description.
2. Add links to Android API doc.
3. Correct some typos.

BUG=XWALK-1620
Merge pull request #1898 from yongsheng/refinedoc

[Android] Format the description, add links and correct typos.