📝 Update Snapshot documentation

[alecgibson]

Snapshot.type should be marked as a string, since it's a
serializable form of a Doc, and the serializable form of a type is its
URI.

The snapshot.type is assumed to be a URI at many points in the code:

• sharedb/lib/projections.js

  Line 82 in e29936c

  if (snapshot.type && snapshot.type !== json0.uri) {

• sharedb/lib/ot.js

  Line 80 in 99ea28e

  snapshot.type = type.uri;

• sharedb/lib/agent.js

  Lines 562 to 563 in e29936c

  	if (types.defaultType !== types.map[snapshot.type]) {
  	data.type = snapshot.type;

[coveralls]

Coverage remained the same at 97.373% when pulling ebb38a0 on snapshot-docs into 916705a on master.