[llvm] [Docs] Improve Target TableGen Docs (PR #178518)

[Craig Topper via llvm-commits]

================
@@ -828,8 +873,15 @@ class PseudoInstExpansion<dag Result> {
 
 /// Predicates - These are extra conditionals which are turned into instruction
 /// selector matching code. Currently each predicate is just a string.
-class Predicate<string cond> {
-  string CondString = cond;
+class Predicate<code cond> {
+
+  /// This is a C++ expression which can be evaluated to return a bool,
+  /// representing whether the predicate is satisifed.
----------------
topperc wrote:

```suggestion
  /// representing whether the predicate is satisfied.
```

https://github.com/llvm/llvm-project/pull/178518