(cleanup) Updated preamble.js

##### This commit makes the following changes:
- Note EXPORTED_FUNCTIONS when saying that mangled functions fail
- Reference malloc as _malloc, as this is how it is called form Module
- Fix formatting in setValue/getValue
- Change array buffer to ArrayBuffer
- Remove outdated note to link TYPED ARRAYS MODE2 DOCUMENTATION

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I'm not sure if this is the right place to contribute to these docs. If it is not, could you point me to the right place? I couldn't find the `site/` mentioned by the `Readme`.

Also, I'd love to help out more in this project. Is it appropriate to be looking at the docs? Is there somewhere I can be of more use, having just been introduced to the project?