Write guidelines for how to talk about new features

[JJ]

The problem

After introducing allocate in the documentation, #2132 found it didn't work, making people confused.

Suggestions

I would suggest inserting something in the writing guidelines to the tune of:

• If you are adding a new feature recently introduced in Rakudo, please indicate in a note which version was it introduced in.
• Try to introduce only documentation for Star versions. Non-star versions should probably wait.
• If you change an example to use the new feature, leave the old example (at least while it's not obsolete) for people who has not upgraded yet.

What do you think? Any more suggestions? This is related to #302 , but that would be major. This, however, would avoid confusion.