python3: unexplained @ thing (convention?) introduced

Something, I don't know what, that begins with a snail magically appears in this document ( .https://learnxinyminutes.com/docs/python3/ ) section 6 with no explanation:-

```
# A class method is shared among all instances
# They are called with the calling class as the first argument
@classmethod
def get_species(cls):
return cls.species
```
What is "@classmethod", and what does it do, and what other things can go there? If it's just a do-nothing comment, we should be told such things exist someplace, and it shouldn't be all-lower-case in that situation.

Start from the Python 3 document at https://learnxinyminutes.com/docs/python3/, section 6, around the @classmethod example. Read the surrounding class-method text and add enough explanation that the @ decorator syntax and @classmethod are introduced before use. Done means a reader can tell @classmethod is not a comment and understands what kind of construct can appear there.

Scritto dal modello di indicizzazione a partire dal testo della issue.