Improve password reset documentation

[clochix]

In the reset password email, we says “Don't forget to update all your encrypted data afterwards!”. I think most users won't understand what this mean, so we should add an explanation somewhere inside the documentation. (there's already some documentation inside the developer doc. but it's not end user oriented).

Once it has be done, please open a new issue inside cozy/cozy-proxy to add the url of this documentation to the reset password documentation.