Update slice2swift doc-comment generation

[bernardnormier]

See https://www.swift.org/documentation/docc/writing-symbol-documentation-in-your-source-files

The doc-comment we currently generate are not quite correct:

• parameter/returns should be capitalized
• parameter should not list the parameter type
• there are no many blank lines
• we should use the Parameters style when there are more than 2 parameters