Not sure "splain" is the precise right word here - it covers the tone, but not the selection of which bit to answer - but I know the behaviour you mean. It's part of why asking a question of my geekosphere requires predicting and heading off said behaviour in advance. It's one of the least offensive of the cluster of obnoxious geek behaviours I think of as "Slashdot-style argumentation".
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.