[ruby/tmpdir] [DOC] Improve documentation

https://github.com/ruby/tmpdir/commit/b9c880f2b6

lib/tmpdir.rb

@@ -108,6 +108,7 @@ class Dir
     end
   end
 
+  # Temporary name generator
   module Tmpname # :nodoc:
     module_function
 
@@ -115,16 +116,23 @@ class Dir
       Dir.tmpdir
     end
 
+    # Unusable characters as path name
     UNUSABLE_CHARS = "^,-.0-9A-Z_a-z~"
 
-    class << (RANDOM = Random.new)
+    # Dedicated random number generator
+    RANDOM = Random.new
+    class << Random # :nodoc:
+      # Maximum random number
       MAX = 36**6 # < 0x100000000
+
+      # Returns new random string upto 6 bytes
       def next
         rand(MAX).to_s(36)
       end
     end
     private_constant :RANDOM
 
+    # Generates and yields random names to create a temporary name
     def create(basename, tmpdir=nil, max_try: nil, **opts)
       origdir = tmpdir
       tmpdir ||= tmpdir()