Add user documentation for abstract data interfaces (open-mmlab#9)

* update isort

* update

* update comments

* update

* update

* update

* fix bug in use cases

* resolve all comments

* add description of basedatasample

* rename meta_info to metainfo as meta is an adj.

* update

* resolve comments

* clean set data

* finished doc

* rephrase

* update

* resolve comments

* more clear organization

* rephrase

* rephrase

* replace 基础数据元素 by 数据元素

* rephrase