Allow multiple aliases in one doc(alias) attribute

[jplatte]

It would be nice to be able to specify multiple aliases in one #[doc(alias)] attribute (as an array). This would be particularily useful for crates that export multiple derive macros with different sets of attributes. With #[doc(alias)], these can make the derive macro findable by any of its attributes names, e.g. serde could do sth. like

#[doc(alias = "default")]
#[doc(alias = "deny_unknown_fields")]
#[doc(alias = "untagged")]
#[doc(alias = "...")]
pub trait Deserialize<'de>: Sized { /* ... */ }

#[doc(alias = "into")]
#[doc(alias = "getter")]
#[doc(alias = "untagged")]
#[doc(alias = "...")]
pub trait Serialize { /* ... */ }

but needing one attribute / line for every attribute gets a bit unwieldy. This would be better:

#[doc(alias = ["default", "deny_unknown_fields", "untagged", "...")]
pub trait Deserialize<'de>: Sized { /* ... */ }

#[doc(alias = ["into", "getter", "untagged", "..."])]
pub trait Serialize { /* ... */ }

[GuillaumeGomez]

From the attribute syntax I can see from https://doc.rust-lang.org/reference/attributes.html#meta-item-attribute-syntax, the [] syntax doesn't seem like a good idea. Instead I simply suggest:

#[doc(alias("into", "getter", "untagged", "..."))]
#[doc(alias = "into")]
pub trait Serialize { /* ... */ }

Accepting both syntaxes seems like a good compromise. Well, I'll open a PR and we can debate over the syntax there.