Documentation checklist

The aim here is to have a checklist of documentation suggestions, and also to keep track, in case others have suggestions or are already working on one, they can reference from here. Contributions/suggestions on what should be included as documentation are welcome.

# Content
- [x] #2679
- [x] #2611
- [x] #2400
- [x] #2665
- [x] add examples to functions in docstrings; this will help users understand how to use the functions quickly,
- [x] transformation documentation
- [x] Create a column/multiple columns
- [x] Mutate existing columns
- [x] Operation between columns
- [x] Apply function across columns (row-wise and column-wise)
- [x] Iteration through a frame
- [x] Sorting on a Frame
~- [ ] Aggregations (with and without groupby) - refer to `by` documentation~
- [x] Datatable rules on column names during transformation
~- [ ] Combining columns/frames (rbind/cbind)~
~- [ ] Comparing frames~
- [x] Conditional transformations

- [x] Selecting and Filtering data (`i` and `j`)
- [x] Selection by label/position/callable/type? (``j``)
- [x] notes/warnings on combining selection options
- [x] Filtering/selecting rows in the ``i`` section (position/callable/type?)
- [x] Drop rows/columns
- [x] Selecting with missing labels
- [x] Warning on use of python keywords for column names
- [x] Single value access
- [x] Boolean indexing
- [x] Bracket notation vs attribute selection with `f`?
- [x] Note on irrelevance of index labels?

- [ ] documentation on joins
- [ ] Combining Frames horizontally and vertically (cbind/rbind)
- [ ] Combining Frames in SQL-like manner (left join)
- [ ] Limitations of SQL-like join
- [ ] Comparing Frames
- [ ] string operations in datatable
- [ ] cookbook
- [ ] updates to existing documentation (e.g fread ... more information on fread and excel (named ranges, ...)
- [ ] iread examples #3353
- [x] Document all functions in the API: #2586

# Technical
- [x] #2605
- [x] #2625
- [x] #2626
- [x] #2627
- [x] #2628
- [x] #2633
- [x] #2654