docs/library/sys.rst: Document implementation.version.releaselevel.

Signed-off-by: Jos Verlinde <jos_verlinde@hotmail.com>

docs/library/sys.rst

@@ -69,14 +69,17 @@ Constants
    MicroPython, it has following attributes:
 
    * *name* - string "micropython"
-   * *version* - tuple (major, minor, micro), e.g. (1, 7, 0)
+   * *version* - tuple (major, minor, micro, releaselevel), e.g. (1, 22, 0, '')
    * *_machine* - string describing the underlying machine
    * *_mpy* - supported mpy file-format version (optional attribute)
 
    This object is the recommended way to distinguish MicroPython from other
    Python implementations (note that it still may not exist in the very
    minimal ports).
 
+   Starting with version 1.22.0-preview, the fourth node *releaselevel* in
+   *implementation.version* is either an empty string or ``"preview"``.
+
    .. admonition:: Difference to CPython
       :class: attention