[WIP] Clarify the meaning of the term "same autotile".

_[This issue is WIP, hope to add more detail/aspects.]_

**Your Godot version:**

3.2.3.official

**Issue description:**

It would be helpful to clarify the meaning of the phrase "must be filled using the same autotile" as used here (and similar places):

https://github.com/godotengine/godot-docs/blob/93679aa6c995c2972ed4692663e2ebf9d8f86c0b/tutorials/2d/using_tilemaps.rst#L199-L200

I didn't really understand the term until I read this section [here](https://docs.godotengine.org/en/latest/tutorials/2d/using_tilemaps.html#autotile-binding):

> _By default, autotile only checks for adjacent cells filled using the same autotile. This behaviour can be overridden in order to have autotiles bind to each other, or even bind to empty cells._

Part of the confusion is because "autotile" sounds like a singular term but it really means "the *combination* of a set of tiles & rules for choosing between them"--but this doesn't seem to be explicitly clarified.

Also:

* The "2x2 mode can only match cells that are part of a 2-by-2 block - cells with no neighbors and lines only one cell wide are not supported." limitation would be more usefully explained up front.

* Current wording is extremely confusing & difficult to understand, e.g.:

> Where a bit is "on", all cells connected to that corner must be filled using the same autotile, in order for the bitmask to match. For example, if the top-left bit is set, the cell directly above, directly left, and diagonally above-left must be filled.
>
> Where a bit is "off", at least one cell connected to that corner must not be set using the same autotile.

* Would be a better UX if the 2x2 template had the "blank" tile in the default icon slot.

**URL to the documentation page (if already existing):**