Docs should warn about tile artifacts due to import filtering

**Godot version:**
3.1

**Page:**
https://docs.godotengine.org/en/3.1/getting_started/workflow/assets/importing_images.html#filter

**Issue description:**
A issue that has been reported here several times is one of edge artifacts on tilemap tiles. Usually the suggestion is to switch on Pixel Snap. However this isn't always appropriate and it doesn't solve the issue if it is due to filtering when the tileset image was imported.

**Suggestion:**
Append to the section **Filter** something like:
> We recommend disabling the filter when importing 2D spritemap or tileset images, as otherwise the tilemaps may suffer artifacts at the boundaries between tiles.

_(Apologies if I should have posted docs-related issues somewhere else)_

Open the linked importing images page for Godot 3.1 and locate the Filter section. Add the requested warning about disabling filtering for 2D spritemap or tileset images, then verify the rendered documentation and wording in context.

Written by the indexing model from the issue text.