[docs] [tblgen] There is no "code" type.

Code fragments are just fancy string literals.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201034 91177308-0d34-0410-b5e6-96231b3b80d8
[docs] [tblgen] clarify that code fragments are just string literals

Fun fact: looking at the TableGen code (around TGParser.cpp:1166), the
only difference in handling is that adjacent regular string literals are
concatenated in the parser.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201035 91177308-0d34-0410-b5e6-96231b3b80d8