docfix

Change-Id: I4c0253856e1a82b4cf037a5224e97e6a9db88be6
Update doc.go
Add way to add data to existing classes.

This lets external packages add their own locally scoped values to
classes that are already created.
Merge pull request #1 from spacemonkeygo/feature-add-data

Add way to add data to existing classes.
Remove parent walking in MustAddData

Turns out, it’s useful to have children overwrite parent values.
nil error class shouldn't panic on String
expose a way to get data from error classes