message: Update registry documentation with new proc-macro field.
diff: @@ -239,7 +239,11 @@ explaining the format of the entry.
  "yanked": false,
  // The `links` string value from the package's manifest, or null if not
  // specified. This field is optional and defaults to null.
  - "links": null
  + "links": null,
  + // This is `true` if the package is a proc-macro.
  + // Note: This field was added in Rust 1.44. Packages published with
  + // earlier versions will not set this field.
  + "pm": false,
  }
  ```
  @@ -404,6 +408,10 @@ considered as an exhaustive list of restrictions [crates.io] imposes.
  // The `links` string value from the package's manifest, or null if not
  // specified. This field is optional and defaults to null.
  "links": null,
  + // This is `true` if the package is a proc-macro.
  + // Note: This field was added in Rust 1.44. Packages published with
  + // earlier versions will not set this field.
  + "proc_macro": false,
  }
  ```
  