docs(_comp_compgen): note _comp_compgen_NAME vs _comp_compgen NAME

Author: akinomyoga

bash_completion

@@ -437,6 +437,11 @@ _comp_split()
 #   [-alR|-v arr|-c cur] to the child calls of `_comp_compgen` in
 #   `_comp_compgen_NAME`.
 #
+# @remarks When no options are supplied to _comp_compgen, `_comp_compgen NAME
+# args` is equivalent to the direct call `_comp_compgen_NAME args`.  As the
+# direct call is slightly more efficient, the direct call is preferred over
+# calling it through `_comp_compgen`.
+#
 # @remarks Design `_comp_compgen_NAME`: a function that produce completions can
 # be defined with the name _comp_compgen_NAME.  The function is supposed to
 # generate completions by calling `_comp_compgen`.  To reflect the options