chore(doc): add note to better perf option

Author: sainnhe

doc/sonokai.txt

@@ -499,6 +499,9 @@ of milliseconds.
     Available values:   `0`, `1`
     Default value:      `0`
 
+Note: the initial generation of syntax files, which occurs on the first Vim
+restart after enabling this option, requires up to 5 seconds to complete.
+
 ------------------------------------------------------------------------------
                                                      *g:sonokai_colors_override*
 g:sonokai_colors_override~