Improvements to TableGen/LangIntro.rst

Document the 'code' data type, and that value{15-17} is different to 
value{17-15}.

Patch by @chenwj (Wei-Ren Chen).

Differential Revision: https://reviews.llvm.org/D32117

llvm-svn: 301920

llvm/docs/TableGen/LangIntro.rst

@@ -58,6 +58,10 @@ types are:
     The 'string' type represents an ordered sequence of characters of arbitrary
     length.
 
+``code``
+    The `code` type represents a code fragment, which can be single/multi-line
+    string literal.
+
 ``bits<n>``
     A 'bits' type is an arbitrary, but fixed, size integer that is broken up
     into individual bits.  This type is useful because it can handle some bits
@@ -105,7 +109,7 @@ supported include:
     hexadecimal integer value
 
 ``"foo"``
-    string value
+    a single-line string value, can be assigned to ``string`` or ``code`` variable.
 
 ``[{ ... }]``
     usually called a "code fragment", but is just a multiline string literal
@@ -126,7 +130,8 @@ supported include:
     access to one bit of a value
 
 ``value{15-17}``
-    access to multiple bits of a value
+    access to an ordered sequence of bits of a value, in particular ``value{15-17}``
+    produces an order that is the reverse of ``value{17-15}``.
 
 ``DEF``
     reference to a record definition