Add attributes to override doc comments for every component of the generated code

[Veetaha]

The motivation for this feature is this reddit comment.

I propose the following attributes design:

Structs

/// Struct docs (not copied anywhere)
#[derive(bon::Builder)]
#[builder(
    builder_type(docs(
        /// Builder type docs
    )),
    start_fn(docs(
        /// Starting function docs
    )),
    finish_fn(docs(
        /// Finishing function docs
    ))
)]
struct Example {
    /// Field docs (inherited by setter by default)
    #[builder(docs(
        /// Setter docs override
    ))]
    field: i32,
}

Functions

/// Start fn docs
#[builder(
    builder_type(docs(
        /// Builder type docs
    )),
    finish_fn(docs(
        /// Finishing function docs
    ))
)]
fn example(
    /// Setter docs (always moved to the setter).
    /// Rust doesn't support docs on arguments directly otherwise
    arg: u32
) {}

A note for the community from the maintainers

Please vote on this issue by adding a 👍 reaction to help the maintainers with prioritizing it. You may add a comment describing your real use case related to this issue for us to better understand the problem domain.

[Veetaha]

Closed by #145. This feature still requires some documentation and pending the 3.0 release, but that is tracked separately in #156