Documented the removal of alignment in version 4 bytecode files (version 3

Documented the removal of alignment in version 4 bytecode files (version 3
was released with LLVM 1.3). Alignment of fields can add severe size to
certain kinds of functions (those with lots of long format instructions) so
we decided to do away with alignment altogether in this bc version.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@15874 91177308-0d34-0410-b5e6-96231b3b80d8