Go to Telegram Settings → scroll down to the version number → press and hold, release and again press and hold it. A small menu will appear.
This option will manually re-import all your phone contacts.
This reloads your contacts which are already imported in server. This normally happens every 24 hours automatically.
This will remove all the uploaded unregistered numbers from the server, and then uploads the current phonebook from the device. Perfect, in case you logged in with your account on a friend's device and contacts mixed up.
When used, it will clear all the chats contents and downloading them again from the server. Useful in case some dialogs are missing in the client.
On startup, the app is only checking for available cameras, it doesn't do anything with it. Unfortunately there are cameras with an objective (e.g. this one) and people are afraid that it could damage the camera. Disabling the function disables the checks but of course also disables the in-app camera.