Explain the casing/substring matching of Enums on help-msgs and errors

[ankostis]

From the help-messages, config-file headers and invalid-value error-messages on FuzzyEnum it's impossible to know whether it is case-sensitive and matches substrings or prefixes.
That was problematic also because allow_none information was simply omitted.

Changes:

• Help and Config messages reuse Enum.info() instead of Enum.values (as Trait.error(0 does).
• Enhanced CaselessStrEnum.info().
• Changes possibly(?) relevant also for Better Errors for Nested Traits #402.

Sample output:

• Before:

  refact(help): reuse `Enum.info()` 
  
  --TstampSender.send_transfer_encoding=<FuzzyEnum>
      Set the Content-Transfer-Encoding MIME Header and encodes appropriately
      outgoing mails.
      Choices: ['BASE64', 'QUOPRI', '7OR8BIT', 'base64', 'quopri', '7or8bit', 'noenc']
      Default: None
  

• AFTER:

  --TstampSender.send_transfer_encoding=<FuzzyEnum>
      Set the Content-Transfer-Encoding MIME Header and encodes appropriately
      outgoing mails.
      Choices: any case-sensitive prefix of ['BASE64', 'QUOPRI', '7OR8BIT', 'base64', 'quopri', '7or8bit', 'noenc'] or None
      Default: None
  

[ankostis]

I think also class_config_rst_doc() does not report if none_allowed.