Update semantics to document scala.Symbol support

Also document that they are a potential source of memory leaks.

Fixes #23
References scala-js/scala-js#234
Document impossibility of catching java.lang.StackOverflowError

Fixes #22
References scala-js/scala-js#212
Document missing support for character sets

Fixes #15
References scala-js/scala-js#177
Change absolute URLs to relative URLs to ease local testing
Merge pull request #24 from jonas/scala-symbol-support

Update doc/semantics to document newly added scala.Symbol support and missing charset and StackOverflowError