More docstrings in the sampling code (#3778)

* add docstring to _choose_backend
* docstring in singleprocess sampling call tree
* docstrings in population sampling
* more accurate docstring/comment on population sampling
* correct order of arguments
* take out creation of unused MultiTrace
* type hints and docstring for _mp_sample
* remove spaces before colons in docstrings to make sphinx happy

Co-authored-by: Colin <[email protected]>

Author: michaelosthege