FIX #1093446. Description of allow_wraparound was incorrect.

In addition to fixing the problem, the patch also
improves readibility of other sentences a little.
FIX #1096857. Update reference to FIPS 180-4.
A set of small changes to documentation.

* Add table to Crypto.Util package docs
* Clarify that PKCS#1v1.5 encryption only works on byte strings
* Clarify that padding is ignored by Cipher classes
* Clarify that block encrypt() and decrypt() do not respectively
  add and remove any padding.
* Clarify what the 'overflow' parameter does (that is, nothing)
  to the Crypto.Util.Counter class.