[#156] Document evaluation of non-strings

apheleia.el

@@ -1034,6 +1034,11 @@ then the first string element of the command list is resolved
 inside node_modules/.bin if such a directory exists anywhere
 above the current `default-directory'.
 
+Any list elements that are not strings and not any of the special
+symbols mentioned above will be evaluated when the formatter is
+invoked, and spliced into the list. A form can evaluate either to
+a string or to a list of strings.
+
 The \"scripts/formatters\" subdirectory of the Apheleia source
 repository is automatically prepended to $PATH (variable
 `exec-path', to be specific) when invoking external formatters.