Better docstring for text_direction

Explains all the algorithms impacted by the setting in blla.segment()
and pageseg.segment().

Author: mittagessen

kraken/blla.py

@@ -260,7 +260,10 @@ def segment(im: PIL.Image.Image,
         im: Input image. The mode can generally be anything but it is possible
             to supply a binarized-input-only model which requires accordingly
             treated images.
-        text_direction: Passed-through value for serialization.serialize.
+        text_direction: Determines principal text direction for heuristic
+                        reading order determination and fallback value for line
+                        orientation in case of low model confidence.
+                        Passed-through value for Segmentation container class.
         mask: A bi-level mask image of the same size as `im` where 0-valued
               regions are ignored for segmentation purposes. Disables column
               detection.

kraken/pageseg.py

@@ -319,8 +319,10 @@ def segment(im: PIL.Image.Image,
 
     Args:
         im: A bi-level page of mode '1' or 'L'
-        text_direction: Principal direction of the text
-                        (horizontal-lr/rl/vertical-lr/rl)
+        text_direction: Determines principal text direction for heuristic
+                        reading order determination and value for line
+                        orientation. Passed-through value for Segmentation
+                        container class.
         scale: Scale of the image. Will be auto-determined if set to `None`.
         maxcolseps: Maximum number of whitespace column separators
         black_colseps: Whether column separators are assumed to be vertical