Improve instructions for Bitwarden DuckDuckGo (DDG) integration

[DenBesten]

@Nail1684

As the context of that link is the browser extension (which is implemented on Mac via the integration between DDG and the BW desktop app), I wouldn’t mention iOS and Android, as it’s also not mentioned for the rest of the browser extensions.

The fact that browser extensions are not needed on mobile browsers is not uniformly known. Hence the reason I propose calling it out here (and perhaps elsewhere too).