Improve docs for noalias and tailcall

Author: CodaFi

Sources/LLVM/Call.swift

@@ -22,8 +22,10 @@ public struct Call: IRInstruction {
     set { LLVMSetInstructionCallConv(self.llvm, newValue.llvm.rawValue) }
   }
 
-  /// Returns whether this function call is a tail call.  That is, if the callee
-  /// may reuse the stack memory of the caller.
+  /// Returns whether this function call is a tail call.
+  ///
+  /// A tail call may not reference memory in the stack frame of the calling
+  /// function.  Therefore, the callee may reuse the stack memory of the caller.
   ///
   /// This attribute requires support from the target architecture.
   public var isTailCall: Bool {

Sources/LLVM/Function+Attributes.swift

@@ -163,6 +163,14 @@ public enum AttributeKind: String {
   /// argument or return value are not also accessed, during the execution of
   /// the function, via pointer values not based on the argument or return
   /// value.
+  ///
+  /// The `noalias` attribute may appear in one of two location: on arguments
+  /// types or on return types. On argument types, `noalias` acts like the
+  /// `restrict` keyword in C and C++ and implies that no other pointer value
+  /// points to this object.  On return types, `noalias` implies that the
+  /// returned pointer is not aliased by any other pointer in the program.
+  ///
+  /// Practically, this allows LLVM to reorder accesses to this memory.
   case noalias
   /// This indicates that the callee does not make any copies of the pointer
   /// that outlive the callee itself.