docs: add missing vim help for priority config (#83)

Author: dmolin

doc/nvim_context_vt.txt

@@ -32,6 +32,10 @@ To customize the behavior use the setup function:
       -- Default: '-->'
       prefix = '',
 
+      -- Override default virtual text priority
+      -- Default: 1000
+      priority = 1000,
+
       -- Override the internal highlight group name
       -- Default: 'ContextVt'
       highlight = 'CustomContextVt',