[IDEA] Clarify the FilterFunc return value functionality

[braydonk]

Thank you for this zap core, it was a big help to myself and my team!

Is your feature request related to a problem? Please describe.
The documentation for FilterFunc does not mention when to return true or false. Because of the language of this being a filtering function, it was confusing to me figuring out whether true meant yes, filter the log, or if it meant yes, send the log through. I had to do some manual testing to figure it out.

Describe the solution you'd like
The documentation for FilterFunc should make some mention what happens under true and false return values from it. Something along the lines of:
"If this function returns true, the log will be let through, otherwise it will be filtered"
(I'm new to zap in general, there may be terminology that makes more sense in this context)