Merge #2406

2406: Move authors assigner to required attributes section of Gem::Specification r=bronzdoc a=greysteil

Currently, the [gem specification docs](https://guides.rubygems.org/specification-reference/) suggest that author / authors is a "recommended" attribute.

In fact, [author / authors is a required attribute](https://github.com/rubygems/rubygems/blob/master/lib/rubygems/specification_policy.rb#L287-L291).

This PR is a code move that puts the `authors` assignment method into the required attributes section (and updates its description to explain that specifying `author` is an alternative). This should make the docs much clearer next time they're generated.

Co-authored-by: Grey Baker <greysteil@gmail.com>

lib/rubygems/specification.rb

@@ -260,22 +260,11 @@ def files
              ].flatten.compact.uniq.sort
   end
 
-  ######################################################################
-  # :section: Recommended gemspec attributes
-
   ##
-  # Singular writer for #authors
+  # A list of authors for this gem.
   #
-  # Usage:
-  #
-  #   spec.author = 'John Jones'
-
-  def author= o
-    self.authors = [o]
-  end
-
-  ##
-  # Sets the list of authors, ensuring it is an array.
+  # Alternatively, a single author can be specified by assigning a string to
+  # `spec.author`
   #
   # Usage:
   #
@@ -285,6 +274,9 @@ def authors= value
     @authors = Array(value).flatten.grep(String)
   end
 
+  ######################################################################
+  # :section: Recommended gemspec attributes
+
   ##
   # A long description of this gem
   #
@@ -403,6 +395,17 @@ def licenses= licenses
   ######################################################################
   # :section: Optional gemspec attributes
 
+  ##
+  # Singular (alternative) writer for #authors
+  #
+  # Usage:
+  #
+  #   spec.author = 'John Jones'
+
+  def author= o
+    self.authors = [o]
+  end
+
   ##
   # The path in the gem for executable scripts.  Usually 'bin'
   #