You can't find it in the manual, as it isn't mentioned there. It is a limit on the amount of techs you can research in one turn due to high research output. The formula is N+1, where N is the number of planets you have. I.e., if you have only your homeworld you can research 2 (1+1) techs in one turn.
Keep in mind, however, that you can only research several techs in one turn, if they are in line after your current research project. If you have already reached the last tech in a line, any research point you don't need to finish the research will be wasted.
Also, it's just plain N in DL, at least last time I checked.
That second part is not quite true. While you can't use EOL overflow in the same turn (without an anomaly, anyway), EOL (end-of-line) techs do provide overflow beyond what they cost. The formula is strange at best, though-at a minimum, you'll lose 25% of your research output for that turn, but there seems to be some other factor at some point that I haven't nailed down yet.
Let's say I'm researching Terraforming, (xml cost 3000, Very Fast/Gigantic modifier roughly 0.6, total cost before inflation 1800), and for simplicity's sake pretend I don't have any tp invested in it yet. If I have a research output (note: this is different from research cost, because half of bonus production is free) of 2400, then I will get no overflow from this tech's turn on my next turn. If however I have say 2800 output, then I'll get 2800*0.75=2100-1800=300 overflow on the tech I choose on the next turn.