[Docs] Fix minor issues in AdvancedBuilds documentation

This patch modifies the commands under the Bootstrap builds section so
that they include the -DLLVM_ENABLE_PROJECTS flag with the value "clang"
so that the Clang build will actually get picked up. Without this patch
they error out as the CLANG_BOOTSTRAP_PASSTHROUGH variable is processed
in /clang/CMakeLists.txt which isn't picked up without the
LLVM_ENABLE_PROJECTS variable being set appropriately.

This patch also changes any remaining dangling <path to source>
references to <path to source>/llvm to better match the rest of the
file.

Reviewed By: thieta

Differential Revision: https://reviews.llvm.org/D148451