Add documentation for date picker keyboard shortcut

We recently ran into an issue where a user was trying to use // to bring up the date picker in the Confluence 5.9.12. It was not working, but other keyboard shortcuts such as using an @mention would work. After some troubleshooting, we found that the user had disabled their Autoformatting preference, which is what prevented the // from working.

We thought it might be a good idea to include documentation on this keyboard shortcut. We took a look at the Keyboard Shortcuts help screen under the Editor Autoformatting, but there was no reference to the // command.

Just a suggestion.