Improve documentation #225

In general

• Clarify that only ports of similar (define) types can be connected
• Clarify implicit-connect and tag semantics

For library-writers

• Define adapter types - perhaps as a cheatsheet
• General library design philosophy
  • How to choose what port type to use, when to use Passive ports
  • How to choose which level-of-abstraction blocks to use, eg Resistor vs. PulldownResistor, Capacitor vs. DecouplingCapacitor