Add Text Shader Documentation

**Describe the project you are working on:**
https://twitter.com/ZeroPointGame

**Describe the problem or limitation you are having in your project:**
There's no documentation for canvas item shaders applied to texts.

**Describe the feature / enhancement and how it helps to overcome the problem or limitation:**
Documentation about text shaders allows people add nice effects in dynamic fonts.

**Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:**
By what i found back engendering it, texts are rendered as planes and applied characters as textures. All colors like shadow are applied as Vertex Colors. So this should be a minimum canvas item for texts:

```
shader_type canvas_item;
void fragment(){
COLOR = texture(TEXTURE,UV) * COLOR;
}
```

An this is using gradient textures in text. There's not consistence in the plane's uv since it's mapped on a atlas of characters (i think) but you can use the screen UV

```
shader_type canvas_item;

uniform sampler2D gradient;

void fragment(){
vec4 col = texture(gradient,SCREEN_UV.yx);
COLOR = texture(TEXTURE,UV) * COLOR * col;
}
```

**If this enhancement will not be used often, can it be worked around with a few lines of script?:**
It's a few lines of scripts.

**Is there a reason why this should be core and not an add-on in the asset library?:**
No aplied