field_identifier and variant_identifier container attributes

[tobz1000]

These sections are taken from PR #96, where they were initially rejected, because they are not particularly useful for automatic derive.

I'm submitting them again for consideration, for a few reasons:

• This comment makes mention of (ab)using field_identifier without understanding its purpose. Putting aside whether this should be done, it would be helpful for people who come across it during experimentation to understand it.
  • Possibly as a result of the above issue, field_identifier is mentioned in that crate's docs. This is where I found myself in need of a quick refresher on what this attribute actually does.
• So far, there is no other mention of these attributes in the documentation.
• I believe it makes sense to have an exhaustive listing of possible attributes somewhere. Having that list across the "attributes" pages makes sense enough.

I also had a quick look at the history of serde_derive/.../attr.rs, and it seems like there are no other undocumented attributes to add at the moment.