Improve document of initialize_copy

[midnight-wonderer]

I wish there were these notes when I read the document.
The functionality is useful, especially for Digest and HMAC stuff.

I hope more people notice this somewhat hidden functionality.

[rhenium]

Good catch! The empty docs are wrong, and this seems to be a side effect of commit 19cc58d.

However, initialize_copy shouldn't appear in the generated docs since it's a private method. It seems most of Ruby core methods use :nodoc: tag to hide it (for example, https://github.com/ruby/ruby/blob/97c133a8591495156f46b53c613abee8c7088a04/struct.c#L1120-L1136). Could you make it follow the convention?

I wish it were hidden by default in RDoc, though. A quick grep through RDoc's source shows no mention of the special method names like initialize_copy or respond_to_missing?, so it's probably mistakenly assuming the method to be public.

[midnight-wonderer]

Main

Okay, the commit has been updated to follow the convention.

Fluff

It was a happy accident for me, though.
Without the unintended side effects, I would not know that HMAC is dup-safe.

I assumed dup wouldn't work, or at least not the way people intended since this is a native extension.
This opens the possibility of having pre-initialized HMAC without holding onto the raw secret key.

I wish I could add some notes on Digest and HMAC pages.
But maybe, in the future, people who care will be directed here by search engine algorithms already.

[rhenium]

Thanks!