Document @deprecated decorator

[ryyppy]

Probably should be put in the Attribute (Decorator) section

[bdunn313]

Got one out in the wild for this one. I could see taking a different approach here by having a section specifically on deprecation and how language server integrations can show them as having a strikethrough etc like in the vscode extension, but I figured that route should be discussed first.

This is the basic implementation where it's just added as another example to the existing block of attribute examples.