Document how to interpret mesh vertex count in the editor

*Edit by @akien-mga:*

This was originally filed as a bug report but it's a documentation issue. See @clayjohn's writeup at https://github.com/godotengine/godot-docs/issues/4400#issuecomment-731634223 which needs to be integrated somehow in the docs.

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**Godot version:**
3.2.3.stable.official

**OS/device including version:**
Windows 10 Pro Build 19041
GLES3, GTX970

**Issue description:**
Hello fellow Godotians,

I found a nice tree model for a forest, which I'm implementing using MultiMeshInstance. The model (an OBJ) has 50k vertices and 100k faces, as shown by Blender, MeshLab and notepad (I counted vertex def rows). The model imports without a problem and looks exactly as it should, but the performance is abyssmal. 100 of these trees (and nothing else besides that) gives me 30 FPS.


My jaw dropped when I saw **Performance.RENDER_VERTICES_IN_FRAME** values, which is around 200 MILLION. In addition to that, when enabling viewport info in the editor, it shows that the model is around 600k vertices, which is more than 10x than the model on disk! I've tried to export from blender using different formats but nothing changes. The model itself is shown as having more than ten times the amount of vertices, while the amount of rendered vertices if even more, because for 200M vertices each model would have to be made of 2M vertices. I've no idea what's happening here and it's driving me crazy. Please advise.

**Steps to reproduce:**
Just import the attached OBJ and check polycount.
[old_pine_trunk_clean.zip](https://github.com/godotengine/godot/files/5573283/old_pine_trunk_clean.zip)

**Minimal reproduction project:**
Just the model.