Small documentaion update. `nestable metamacros` [skip ci]

[peter1000]

Perhaps escaping should escape the nested calls as well?

I don't have any opinion on that reason:

• escapes will be used very rarely
• current behavour makes it very clear what is escaped and what not
• but escaping the inner would also make sense

[MichaelHatherly]

Yeah, current behaviour can stay for now.