Before revision: Their values a chosen such that the RC time constant is large enough to ensure its cutoff frequency is below the carrier wave's frequency, but low enough so that the capacitor can discharge fast enough to keep up with the envelope when the envelope is falling.
After revision: Their values a chosen such that the RC time constant is large enough to ensure its cutoff frequency is below the carrier wave's frequency, while the RC time constant small enough to discharge fast enough well when the envelope is falling.