* lib/rubygems/package.rb:  Ensure digests are generated for signing.
	* test/rubygems/test_gem_package.rb:  Test for the above.

	* lib/rubygems/security/policy.rb:  Ensure digests are present when
	  verifying a gem and match the number of signatures bidirectionally.
	* test/rubygems/test_gem_security_policy.rb:  Test for the above.

	* lib/rubygems.rb:  Documentation improvements (by zzak)


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39126 b2dd03c8-39d4-4d8f-98ff-823fe69b080e