Update "form" explaination in Glossary

Through recent conversation, it seems like there can be a more clear explanation for the word `form`.

https://opendatakit.slack.com/archives/C5KCRU1L1/p1521481777000547

In current docs, the explanation of a `form` in the [glossary](https://docs.opendatakit.org/glossary/) is stated as `A defined set of questions and answer choices displayed by an application that can render forms written in the XForm standard.`.
According to me, the correct word should be `blank form`.
I propose to add the explanation as follows:

`form`: An XML document written in the XForm standard.

- `blank form`: Stores information about the type of questions, constraints on answers and options (if any) for each question which is rendered by the application. It is generally referred just as `form` in the documentation.

- `filled form`: Stores the user responses. A filled form may also contain other files like images, audio, video.

Start with the glossary page linked in the issue and locate the current definition of `form`. Review the proposed distinctions between `form`, `blank form`, and `filled form`, then update the glossary so those definitions are represented clearly. Done means the glossary distinguishes the XML form document, its question definition, and submitted responses.

Written by the indexing model from the issue text.