Document what tcc_fileextension does

This is evident, but won't hurt

libtcc.c

@@ -168,6 +168,10 @@ PUB_FUNC char *tcc_basename(const char *name)
     return p;
 }
 
+/* extract extension part of a file
+ *
+ * (if no extension, return pointer to end-of-string)
+ */
 PUB_FUNC char *tcc_fileextension (const char *name)
 {
     char *b = tcc_basename(name);