Remove outdated FIXME

This comment predates the addition of $ARGS in jq 1.6, so there was no
way to access named arguments with an invalid identifier. Rather than
forbid such names, update the manual to clarify.

docs/content/manual/dev/manual.yml

@@ -241,7 +241,8 @@ sections:
         bind `$foo` to `"123"`.
 
         Named arguments are also available to the jq program as
-        `$ARGS.named`.
+        `$ARGS.named`. When the name is not a valid identifier, this is
+        the only way to access it.
 
       * `--argjson name JSON-text`:
 

src/main.c

@@ -454,7 +454,6 @@ int main(int argc, char* argv[]) {
           further_args_are_strings = 0;
           further_args_are_json = 1;
         } else if (isoption(&text, 0, "arg", is_short)) {
-          // FIXME: For --arg* we should check that the varname is acceptable
           if (i >= argc - 2) {
             fprintf(stderr, "%s: --arg takes two parameters (e.g. --arg varname value)\n", progname);
             die();