Add non-normative docs for APIs

[inexorabletash]

Several specs (Encoding, DOM, etc) have non-normative documentation for examples, something along the lines of:

store = transaction . objectStore( name )

Gets an IDBObjectStore object.

request = store . add(value, _key)

Adds a record to the store with the specified key. Will fail if the a record with the same key is already present.

request = store . put(value, _key)

Adds a record to the store with the specified key. Will overwrite a previous record with the same key.

... which saves developers from having to read spec-ese, especially as the behavior of the async methods is far away in the spec from the IDL entry points. This is a simplified description that mostly dispenses with error cases, and focuses on the purpose of the methods and intended use.