Documentation: add information about builtin includes to FAQ


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174612 91177308-0d34-0410-b5e6-96231b3b80d8
Documentation: fix typo: stdard -> stdarg


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174613 91177308-0d34-0410-b5e6-96231b3b80d8