fix(help): finish #381 for RsT to print enum-choices

[ankostis]

#381 inserted a new line Choices: x, y, z into the help and config messages for all Enum traits, immediately above Default: ... line.
As had been mentioned in that PR, that feature was never added to the RsT documentation produced by Configurable.class_config_rst_doc() classs method.
This PR fixes that neglect.

• Had to add an optional as_rst=false kwd to allXxxEnum.info() methods.
• Added TCs for info().

RsT samples for info() method:

• Enum & UseEnum traits:

  any of ``'red'``|``'Green'``|``'BLUE'``|``'YeLLoW'`` or `None`
  

• CaselessStrEnum:

  any of ``'red'``|``'Green'``|``'BLUE'``|``'YeLLoW'`` (case-insensitive) or `None`
  

• FuzzyEnum:

  any case-insensitive prefix of ``'red'``|``'Green'``|``'BLUE'``|``'YeLLoW'`` or `None`
  

[minrk]

Adding an as_rst arg is a backward-incompatible change because user traits can have defined .info() accepting no arguments, which must not result in any errors.

Maybe better to define a new .info_rst() method that calls .info() by default.

[ankostis]

ok