Add missing documentation for from_bits() and into_bits()

[osteffenrh]

Fix warnings about missing documentation if #![warn(missing_docs)] is enabled.

For example:

warning: missing documentation for a method
   --> xxx/src/lib.rs:654:1
    |
654 | #[bitfield(u32)]
    | ^^^^^^^^^^^^^^^^ in this procedural macro expansion
    |
   ::: /home/xxx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bitfield-struct-0.6.1/src/lib.rs:39:1
    |
39  | pub fn bitfield(args: pc::TokenStream, input: pc::TokenStream) -> pc::TokenStream {
    | --------------------------------------------------------------------------------- in this expansion of `#[bitfield]`

[wrenger]

Ah, thanks for catching and documenting this.