Document how to rotate screen when not PiTFT

[imrehg]

I.e. how to use lcd_rotate, display_lcd_rotate, display_hdmi_rotate and other configs in config.txt, as user was confused (tried to use the TFT_ROTATE that is not the right one for that role.

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[gjreasoner]

I ran into this on a raspberry pi 4 and a 7" touchscreen, was able to use this device configuration setting to rotate the screen.

Would be great to have on the documentation if this project is still active.