Document cbrain specific boutiques properties (extensions) #1128

[MontrealSergiy]

see #1128

[prioux]

This doc is good but it has indentations problems (e.g. line 228) and overall it should match the conventions for RDOC blocks. Try to put your new large comment block at the beginning of a method and generate the rdoc with the rake tasks, and inspect the results with yoru browser. Make sure it looks good!

I would prefer all your new methods that end with "custom" in their names ot START with "custom" instead. E.g. instead of allow_empty_strings_custom, use custom_allow_empty_strings.

[MontrealSergiy]

I'll add a few directives to facilitate automated generation of documentation by rdoc, and, following @natacha-beck suggestion, move the property table into a separate file

[MontrealSergiy]

Tested with rdoc 6.6.2, please check now @prioux @natacha-beck