Explain what table headers mean

A common question 'is what do "Self/Total" mean?'

@petethepig's explained in [Slack](https://pyroscope.slack.com/archives/C01FJRYENPQ/p1654057914911199?thread_ts=1654052490.571829&cid=C01FJRYENPQ):
> **self** means the amount of time spent within a function, excluding delegations to other functions, and **total** means the amount of time spent within a function including all of the delegations. For example, for a function foo that does work for 2 seconds, then calls function bar that takes 8 seconds, self will be 2 seconds and total will be 10 seconds.

To make things clearer, we could add a tooltip similar to https://github.com/pyroscope-io/pyroscope/pull/1210