Update PatchedBase doc with patch ceiling in spec

The documentation at https://orc.apache.org/specification/ORCv1/ fails to mention that in RLEv2 patched base mode, the bit width of PW + PGW is to be ceiled to the nearest fixed bit size. The values in the fixed bit size set appear to be from the table here https://github.com/apache/orc/blob/9c65bc116a2208d5d729687b7744b2ffbd72fea0/c%2B%2B/src/RLEV2Util.cc#L30-L37 which does not exactly match the values in the 5 bit width encoding table as the latter lacks support for sizes 22 and 23.