Update the PCH internals documentation to cover chained precompiled

headers and modules in more detail. I'd still like to expand on some
of the modules-related issues further, but this is a decent start.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@163989 91177308-0d34-0410-b5e6-96231b3b80d8