DOC: improve documentation for Series.resample
Reference docs (see: http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.resample.html):
- [ ] Add to docs the list of options that can be passed to `fill_method`. Some of which are `pad`, `bfill`, `ffill`.
  -- The list seems to be documented for the `method` on `fillna`: http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.fillna.html#pandas.DataFrame.fillna
- [x] The `convention` parameter is not well documented. It is unclear what the values of `end` or `start` means (and the associated aliases). (#15432 is dupe), PR #16965
- [x] The `kind` parameters is also unclear. The best I could get from it was a infinite stack recursion: #5026. (https://github.com/pandas-dev/pandas/pull/19084)
- [ ] `how` can also be a function or list of functions, not only a string
- [ ] for the `closed` and `label` keywords, I think it could be explained which are default in which cases (which is not always the same). In every case, that `label='right'` is default as stated in the docs (http://pandas.pydata.org/pandas-docs/dev/timeseries.html#up-and-downsampling) is not true.
- [ ] A list of all available methods for `how`

Tutorial docs:
- [ ] example using custom `how` function
- [ ] example showing how to use `base`

  