Various improvements to Camera2D's documentation

A summary of the tweaked descriptions:
* "get_camera_position()" is explains what the camera position is, and mitigates the confusion between this method and Node2D.position;
* "get_camera_screen_center()" links to the "get_camera_position" method;
* "reset_smoothing()" links to the "smoothing_enabled" propriety;

* "drag_horizontal_offset" and "drag_vertical_offset" link to their respective _enabled_ properties;
* "limit_smoothed" links to the "smoothing_enabled" propriety;
* "rotating" clarifies that the camera _view_ rotates when enabled.

Partially closes https://github.com/godotengine/godot-docs/issues/5417, as some of the issues described are no longer present in 4.0, or are not fully solved in this PR.

Further tweaks are welcome, as I'm afraid this could be a too verbose. A few of these could be split into their PR if necessary.

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