Add documentation to the HTML module

[dylandrop]

Title says it all. Add documentation where it is needed.

[bcardiff]

Docs should have some basic example when possible. escape(String) : String and unescape(String) : String are simple enough to add some of them. The IO version can refer to the previous one.

You can find style/inspiration in string.cr .

They don't need to be exhaustive, but representative.

[dylandrop]

@bcardiff No problem -- updated.

[asterite]

Looks good now, thank you!