[lldb][docs] Update documentation for monorepo and CMake caches

Summary: The lldb build system made good progress in the last months, but documentation was still lacking behind. Here's a patch to catch up.

Reviewers: JDevlieghere, jingham, labath, stella.stamenova, teemperor, jryans, kastiglione, xiaobai, compnerd, zturner

Reviewed By: labath, stella.stamenova, jryans

Subscribers: clayborg, amccarth, friss, lldb-commits, #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D65330

llvm-svn: 367302