[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

docs/TableGenFundamentals.rst

@@ -289,7 +289,7 @@ supported include:
     string value
 
 ``[{ ... }]``
-    code fragment
+    usually called a "code fragment", but is just a multiline string literal
 
 ``[ X, Y, Z ]<type>``
     list value.  <type> is the type of the list element and is usually optional.