Document feature flag context example

What does this MR do and why?

Originated from the !128607 (comment 1513602966)

I see people often do:

context 'when FF is enabled' do
end

context 'when FF is disabled' do
end

which leads to bigger diffs and harder-to-review changes.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.