Document Proc#==

Merged: https://github.com/ruby/ruby/pull/3966

proc.c

@@ -1273,6 +1273,32 @@ rb_proc_get_iseq(VALUE self, int *is_proc)
     return NULL;
 }
 
+/* call-seq:
+ *   prc == other -> true or false
+ *   prc.eql?(other) -> true or false
+ *
+ * Two proc are the same if, and only if, they were created from the same code block.
+ *
+ *   def return_block(&block)
+ *     block
+ *   end
+ *
+ *   def pass_block_twice(&block)
+ *     [return_block(&block), return_block(&block)]
+ *   end
+ *
+ *   block1, block2 = pass_block_twice { puts 'test' }
+ *   # Blocks might be instantiated into Proc's lazily, so they may, or may not,
+ *   # be the same object.
+ *   # But they are produced from the same code block, so they are equal
+ *   block1 == block2
+ *   #=> true
+ *
+ *   # Another Proc will never be equal, even if the code is the "same"
+ *   block1 == proc { puts 'test' }
+ *   #=> false
+ *
+ */
 static VALUE
 proc_eq(VALUE self, VALUE other)
 {