[Content] clarify Image to labels, Labels to Image, and add docs for …

…shapes to labels behavior (#414)

# References and relevant issues
Depends on napari/napari#6859

# Description
This PR clarifies the Image to Labels and Labels to Image contextual
menu actions, plus adds documentation for the altered Shapes to Labels
behavior introduced by napari/napari#6859

docs/tutorials/fundamentals/viewer.md

@@ -478,8 +478,8 @@ The right side of the status bar contains some helpful tips depending on which l
 * **Show All Unselected Layers** - Set all *unselected* layers to visible.
 * **Hide All Unselected Layers** - Set all *unselected* layers to hidden.
 * **Duplicate Layer** - creates a second copy of the selected layer. Can be used on **Points**, **Shapes**, **Labels**, and **Image** layers. This is useful for testing your analysis on a copy instead of on the original image.
-* **Convert to Labels** - converts an **Image** layer to a **Labels** layer. This is useful for converting a binary image segmentation map to a labels layer with each segmented object denoted by its own integer. Can also be used on a **Shapes** layer.
-* **Convert to Image** - converts a **Labels** layer into an **Image** layer.
+* **Convert to Labels** - converts an **Image** layer to a **Labels** layer (if the data is already of type `int` it is not copied or altered, only the representation is changed, making this fully reversible). This is useful for converting a binary image segmentation map to a labels layer with each segmented object denoted by its own integer. Can also be used on a **Shapes** layer, in this case a new **Labels** layer will be created.
+* **Convert to Image** - converts a **Labels** layer into an **Image** layer (the data is not copied or altered, only the representation is changed, making this fully reversible).
 * **Convert datatype** - converts a **Labels** layer into int8, int16, int32, int64, uint8, uint16, uint32, or uint64 data types. The initial data type is the data type of the data itself.
 * **Make Projection** - can be used only on a layer with more than 2 dimensions, also known as a *stack*.  It creates a new layer that is a projection of the layer stack with the characteristic the user selects, reducing the number of dimensions by 1. More information about the types of projections is available [here](https://medium.com/@damiandn/an-intoduction-to-biological-image-processing-in-imagej-part-3-stacks-and-stack-projections-942aa789420f). The following projections are available:
     * **Max** - maximum intensity projection. At each pixel position, we go  through the stacks, find the pixel with the maximum intensity, and that becomes the intensity of that pixel value in the projected image.