[docs] Stop advertising 'make update'. It isn't implemented in CMake and

seems unlikely to be added. It also doesn't seem like it should be part
of the build system at all (consider out-of-tree builds).

We should probably add nice, easy tool for this that works both for svn
client trees and git-svn client trees, but it probably won't be spelled
"make update".

llvm-svn: 202430