[LVI] Clarify getValueAt/getValueInBlock doc comments (NFC)

The lattice value returned by getValueInBlock() holds at the start
of the block, not at the end. Also make it clearer what the
difference between getValueInBlock() and getValueAt() is.

Author: nikic

llvm/lib/Analysis/LazyValueInfo.cpp

@@ -434,14 +434,16 @@ class LazyValueInfoImpl {
   void solve();
 
 public:
-  /// This is the query interface to determine the lattice
-  /// value for the specified Value* at the end of the specified block.
+  /// This is the query interface to determine the lattice value for the
+  /// specified Value* at the context instruction (if specified) or at the
+  /// start of the block.
   ValueLatticeElement getValueInBlock(Value *V, BasicBlock *BB,
                                       Instruction *CxtI = nullptr);
 
-  /// This is the query interface to determine the lattice
-  /// value for the specified Value* at the specified instruction (generally
-  /// from an assume intrinsic).
+  /// This is the query interface to determine the lattice value for the
+  /// specified Value* at the specified instruction using only information
+  /// from assumes/guards and range metadata. Unlike getValueInBlock(), no
+  /// recursive query is performed.
   ValueLatticeElement getValueAt(Value *V, Instruction *CxtI);
 
   /// This is the query interface to determine the lattice