Improve function header comment for create_singleton_array().

Mentioning the caller is neither future-proof nor an adequate substitute
for giving an API specification.  Per gripe from Neha Khatri, though
I changed the patch around some.

Discussion: https://postgr.es/m/CAFO0U+_fS5SRhzq6uPG+4fbERhoA9N2+nPrtvaC9mmeWivxbsA@mail.gmail.com

src/backend/utils/adt/array_userfuncs.c

@@ -455,7 +455,10 @@ array_cat(PG_FUNCTION_ARGS)
 
 
 /*
- * used by text_to_array() in varlena.c
+ * create_singleton_array - make a one-element array
+ *
+ * If desired, the caller can ask for it to be higher than one-dimensional.
+ * Caller's fcinfo must be passed in, as we use fn_extra for caching.
  */
 ArrayType *
 create_singleton_array(FunctionCallInfo fcinfo,