Remove comments from core classes in json/add

[stomar]

(Previously submitted to MRI's bug tracker, as https://bugs.ruby-lang.org/issues/12255).

This patch removes some comments that otherwise would appear in MRI's docs of the affected core classes.

E.g. for Exception

$ ri Exception
Descendants of class Exception are used to [...]

[snipped class documentation]

Exception serialization/deserialization        <- does not belong here!
-------------------------------------------
Class methods:

[snip]

Or see for example http://docs.ruby-lang.org/en/trunk/Exception.html, where the same comment appears just above the "Public Class Methods" section.

[stomar]

Thanks!