[analyzer] Update the checker writer manual with explanation of SVals
and the link to checker callback documentation.

SVal, SymExpr, MemRegion description is a slightly edited version of
Ted's reply to a question on cfe-dev list.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@146048 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed