Merge pull request kivy#2764 from kivy/doc_fmt

remove old doc about fixed fmt, short explanations of fmt param

Author: inclement

kivy/graphics/vertex_instructions.pyx

@@ -324,10 +324,6 @@ cdef class StripMesh(VertexInstruction):
 cdef class Mesh(VertexInstruction):
     '''A 2d mesh.
 
-    The format for vertices is currently fixed but this might change in a future
-    release. Right now, each vertex is described with 2D coordinates (x, y) and
-    a 2D texture coordinate (u, v).
-
     In OpenGL ES 2.0 and in our graphics implementation, you cannot have more
     than 65535 indices.
 
@@ -352,7 +348,20 @@ cdef class Mesh(VertexInstruction):
         `mode`: str
             Mode of the vbo. Check :attr:`mode` for more information. Defaults to
             'points'.
-
+        `fmt`: list
+            The format for vertices, by default, each vertex is described by 2D
+            coordinates (x, y) and 2D texture coordinate (u, v).
+            Each element of the list should be a tuple or list, of the form:
+                (variable_name, size, type)
+            which will allow mapping vertex data to the glsl instructions.
+                [
+                (b'v_pos', 2, b'float'),
+                (b'v_tc', 2, b'float'),
+                ]
+            will allow using
+                attribute vec2 v_pos;
+                attribute vec2 v_tc;
+            in glsl's vertex shader.
     '''
 
     def __init__(self, **kwargs):