Size option in bootstrap_field documentation

[GabeSoler]

Hi, this is a documentation small change in the bootstrap field.

Documentation says to use the words, but it actually works with the shortened form:

"sm" "md" "lg"

---

[in bootstrap fields, current look]

size Controls the size of the rendered div.row through the use of CSS classes.

One of the following values:

    * ``'small'``
    * ``'medium'``
    * ``'large'``