RewriteBuffer::write(): Add a doc comment about non-portable use

The function isn't strictly at fault but there are callers using it
incorrectly, causing crashes with in-place edits of 64KB or larger files on
Windows.

See PR17960 for details.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194972 91177308-0d34-0410-b5e6-96231b3b80d8