[ci skip] Refine Action Mailer guide.
* Wrap 80 words if possible.
* Make attachment hash options more readable. (L239-L243)

  Before:

  ![screenshot 2014-06-11 02 59 54](https://cloud.githubusercontent.com/assets/1000669/3235432/a91d0464-f0d2-11e3-9e4a-6af2b2a83f1a.png)

   After:

  ![screenshot 2014-06-11 02 59 36](https://cloud.githubusercontent.com/assets/1000669/3235428/a47b28dc-f0d2-11e3-806c-cfaedd3f15df.png)

* L619 Wrap "location" in code: `"location"`.
* L628 Use actual section name.
* L697 Remove unnecessary `./` when refer to other guide.
Merge pull request #15618 from JuanitoFatas/doc/action-mailer-guide

[ci skip] Refine Action Mailer guide.