simpletrace: document Analyzer method signatures

Users can inherit from the simpletrace.Analyzer class and receive
callbacks when events of interest occur in a trace file.  The method
signature is a little magic because the timestamp and pid arguments are
optional.  Document this.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20170411095654.18383-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>