DSSM负样本构造问题

在examples/match_model/dssm_negative_sample.md中，提到：训练样本一般只需准备点击（正样本）的样本即可
这里的意思是，召回的正样本，可以是未曝光点击的样本吗？并不只能是曝光且点击的样本？

Read examples/match_model/dssm_negative_sample.md, especially the statement about preparing clicked positive samples. Confirm the intended definition of a positive sample and clarify whether an unexposed click is included; the documentation is done when this distinction is stated explicitly.

Written by the indexing model from the issue text.