Add documentation about the new SBExecutionContext for Python commands feature. This marks the end of rdar://18142728

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@218835 91177308-0d34-0410-b5e6-96231b3b80d8
Add SBExecutionContext.cpp to this file as well, as the comment
in source/API/CMakeLists.txt instructs...


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@218837 91177308-0d34-0410-b5e6-96231b3b80d8