Arrows: added 25% transparency to the highlighted arrows,

...as the focus arrows are supposed to be the brightest.

Added/updated imagemagick_command.txt for both standard and highlighted arrows.

Ran wesnoth-optipng over all arrow images:

Overall statistics (only for files with a smaller recompressed size):

  Original size: 261 KiB on 121 files
 Optimized size: 241 KiB
   Total saving: 19 KiB = 7% decrease

images/arrows/highlighted/imagemagick_command.txt

@@ -0,0 +1,5 @@
+Those files were created from a copy of the "focus" arrows with the following imagemagick command:
+
+mogrify -channel Alpha -evaluate Multiply 0.75 *.png
+
+i.e. they are 20% transparent, as their alpha channel was multiplied by 0.75.

images/arrows/standard/imagemagick_command.txt

@@ -1,5 +1,5 @@
-Those files were created from the non-transparent arrows with the following imagemagick command:
+Those files were created from a copy of the "focus" arrows with the following imagemagick command:
 
-mogrify -channel Alpha -evaluate Divide 2 *
+mogrify -channel Alpha -evaluate Divide 2 *.png
 
 i.e. they are 50% transparent, as their alpha channel was divided by 2.