-If <parameter>a</parameter> is NULL, returns a copy of <parameter>b</parameter>.
-If <parameter>a</parameter> is an FcCharSet object owned by fontconfig that
-cannot be modified, it returns the union of <parameter>a</parameter> and
-<parameter>b</parameter> in a newly-created FcCharSet object.
-Otherwise, adds all chars in <parameter>b</parameter> to <parameter>a</parameter> and returns <parameter>a</parameter>. In other words, this is an in-place versionof FcCharSetUnion.
+Adds all chars in <parameter>b</parameter> to <parameter>a</parameter>.
+In other words, this is an in-place version of FcCharSetUnion.
+If <parameter>changed</parameter> is not NULL, then it returns whether any new
+chars from <parameter>b</parameter> were added to <parameter>a</parameter>.
+Returns FcFalse on failure, either when <parameter>a</parameter> is a constant
+set or from running out of memory.