* studentTTest function description

* welchttest description, links in studentttest

* mannWhitneyUTest description

* Text changed slightly

* Text changed slightly

* Text changed slightly

* Text changed slightly

* - First header is first level.
- sample_index type set to UInt64.
- Return values renamed.

- mannWhitneyUTest:
  - added continuity_correction parameter
  - optional parameters wrapped in braces [ ]
  - description refactored
  - example SQL placed on single line

* Improved formulations (hopefully)
General data type names used (Integer, Float, Decimal)
Removed names from return values

* Fixes

* Optional parameters stated explicitly

* New line at the end of files

* Russian function description

* Files moved in the right folder

* Translated 'Original article' to russian

* Typos are fixed

* fixed alowed parameter value: "two sided" -> "two-sided"
* duplicate word "require" is removed from exception messages

* mannWhitneyUTest description changed
Added Tuple link in return values
"samples from two populations" added in short descriptions

* Russian translation updated

* fix
Co-authored-by: NAlexey Smirnov <alexey-sm@yandex-team.ru>