What to document and how often to document inevitably requires a judgment by the supervisor. But keep in mind that memories are fallible and often selective. It's humanly impossible to remember all the events and outcomes over the course of a year that relate to an individual's performance. Most people, of course, remember more recent or vivid events. Yet, recent time periods may not be representative of an entire performance period.
People are also selective in what's remembered.
For these reasons, it makes sense to maintain ongoing performance documentation. One issue concerning documentation is which method or format is appropriate. Some supervisors set up a file into which they regularly drop performance-related notes. Still others note and retain only the most important critical incidents. Again, the format used to record the documentation is a matter of personal preference.