Editor script documentation update

[Jerakin]

Task:

• Add note of sys. not being accessible
  https://defold.com/manuals/editor-scripts/#editor-script-runtime

• Fix lua syntax issues in the collection code snippet.
  https://defold.com/manuals/editor-scripts/#editing-collections

[Jerakin]

Should really be moved to https://github.com/defold/doc forgot about that repository.