Docs, docs and docs

1. Add more examples, explain all points styled-components explains
2. Explain whats the difference to styled-components
3. Explain whats the difference to glamorous (second popular sc interface, more direct competitor)
4. Explain everything that JSS has in the docs but might be not obvious with styled-jss API.

Start by reviewing the current styled-jss documentation and API, then compare its coverage with styled-components, glamorous, and JSS documentation. Add examples and explanations for the requested differences and less-obvious API behavior. Done means the documented features and comparisons are comprehensive and understandable.

Written by the indexing model from the issue text.