Update time filter docs, clarify presence/absence of fields#143

thomas-nicolai-frequenz merged 3 commits intofrequenz-floss:v0.x.xfrequenz-floss/frequenz-api-reporting:v0.x.xfrom

stefan-brus-frequenz:end_timestefan-brus-frequenz/frequenz-api-reporting:end_timeCopy head branch name to clipboard