Some documentation for _onMetadata

[OlaviSau]

Writing documentation to understand webtorrent ^^

[OlaviSau]

All suggestions open

[feross]

Thanks for sending this in. This PR has a few problems:

1. The tests don't pass. Run npm test to make sure your changes don't break any of the tests.
2. I prefer succinct inline comments. If you wanted to move some of what you wrote inline, that would be fine.
3. You also changed some code, so the PR name is misleading. I think this probably introduces a bug.

[OlaviSau]

Thanks for responding :)
Sorry about the tests, pushed it at like 2 am and forgot about them since i had copy pasted from github , appareantly it put some github gibberish into it ^^ last test only required newlines at the end.
About the comments, yeah whatever you like, I wrote it to understand webtorrent better, so perhaps I can contribute in some larger things rather than minor bugfixes :)
And about the code change, the PR is name is misleading indeed sorry about that, but the code change itself is valid, you don't need to check if the object exists if you are checking a property on it.

The following values are always falsy:

false
0 (zero)
"" (empty string)
null
undefined
NaN (a special Number value meaning Not-a-Number!)

Oh and have you considered seperate documentation from code?