Added a few lines to clear the concept of main and renderer thread(s) by comparing it with a client-server architecture as sometimes understanding this concept becomes difficult for beginners.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Documentation
• Code style update
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Does this PR introduce a breaking change? (check one)

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The PR fulfills these requirements:

• It's submitted to the dev branch and not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• It's been tested on a Cordova (iOS, Android) app
• It's been tested on a Electron app
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If adding a new feature, the PR's description includes:

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