A central place explaining and linking to different types of segments and tasks

The documentation mentions "pending segments" and "pending tasks" here and it seems to me that there is no place where this concept is clearly described and other "segments states" or "segment types" are listed.

Such central doc would be useful not only on the users' side (in users' docs) but in the codebase too. A central place can link to different classes "implementing" or "most relevant" to different types of segments.

FYI @jihoonson

Start by locating the documentation passages that mention "pending segments" and "pending tasks," then identify the relevant segment states, types, and implementing classes in the codebase. Done means a central document clearly describes the concepts and links to the user documentation and most relevant classes.

Written by the indexing model from the issue text.