text: Title: [ENH] provide proper mathematical description in docstring of splitters
  Body: Many of the splitters are poorly described, in terms of formalism, and in terms of parameters. We should write proper formal descriptions of the splitters.
  
  For a complete description, the docstring should describe:
  
  * what exactly the training folds are
  * what exactly the test folds are
  * how many splits there are
  
  Descriptions of the folds should be unambiguous in case of irregular indices (if applicable), i.e., are we getting all indices within a specific interval, or are we getting specific indices?
  
  Further, all parameters should be precisely explained.
  
  An example for this is this PR for `SlidingWindowSplitter`:
  https://github.com/sktime/sktime/pull/7195
  
  Splitters with incomplete description:
  
  - [ ]  `ExpandingCutoffSplitter` #7774 
  - [ ]  `ExpandingGreedySplitter`
  - [ ]  `ExpandingWindowSplitter`
  - [x] `SlidingWindowSplitter`  #7195
  - [x] `SingleWindowSplitter` #7376 
  - [ ]  `TemporalTrainTestSplitter`
  - [ ] `temporal_train_test_split` #7578 