all: change version back to 3.99; docs: warn about know regressions and

clarify GPLv2+ COPYING

doc/upx-doc.txt

@@ -16,7 +16,7 @@ ABSTRACT
     formats supported, because of in-place decompression.
 
 DISCLAIMER
-    UPX comes with ABSOLUTELY NO WARRANTY; for details see the file LICENSE.
+    UPX comes with ABSOLUTELY NO WARRANTY; for details see the file COPYING.
 
     Please report all problems or suggestions to the authors. Thanks.
 
@@ -831,9 +831,15 @@ COPYRIGHT
 
     This program is distributed in the hope that it will be useful, but
     WITHOUT ANY WARRANTY; without even the implied warranty of
-    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the UPX License
-    Agreement for more details.
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 
-    You should have received a copy of the UPX License Agreement along with
-    this program; see the file LICENSE. If not, visit the UPX home page.
+    UPX is distributed with full source code under the terms of the GNU
+    General Public License v2+; either under the pure GPLv2+ (see the file
+    COPYING), or (at your option) under the GPLv+2 with special exceptions
+    and restrictions granting the free usage for commercial programs (see
+    the file LICENSE).
+
+    You should have received a copy of the UPX License Agreements along with
+    this program; see the files COPYING and LICENSE. If not, visit the UPX
+    home page.