[PH2][Documentation][Promise] Seq (#38339)

[PH2][Documentation][Promise] Seq

Closes #38339

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/38339 from tanvi-jagtap:ph2_comb_seq_doc b1d393ab21
PiperOrigin-RevId: 722257003

src/core/lib/promise/seq.h

@@ -28,6 +28,47 @@
 
 namespace grpc_core {
 
+// Seq Promise combinator.
+//
+// Seq stands for sequence.
+//
+// Input :
+// 1. The seq combinator needs minimum one promise as input.
+// 2. The first input to seq combinator is a promise.
+// 3. The remaining inputs to seq combinator are promise factories. The input
+// type of the Nth functor should be the return value of the (N-1)th promise.
+//
+// Return :
+// Polling the Seq Promise combinator returns Poll<T> where T is the type
+// returned by the last promise in the list of input promises.
+//
+// Polling the Seq combinator works in the following way :
+// Run the first promise. If it returns Pending{}, nothing else is executed.
+// If the first promise returns a value, pass this result to the second functor,
+// and run the returned promise. If it returns Pending{}, nothing else is
+// executed. If it returns a value, pass this result to the third, and run the
+// returned promise. etc. Return the final value.
+//
+// If any of the promises in the Seq chain returns a failure status, Seq will
+// still proceed with the execution of the remaining promises. If you want the
+// execution to stop when a failure status is received, use the TrySeq
+// combinator instead.
+//
+// Promises in the Seq combinator are run in order, serially and on the same
+// thread.
+//
+// Example :
+//
+// TEST(SeqTest, TwoThens) {
+//   auto initial = [] { return std::string("a"); };
+//   auto next1 = [](std::string i) { return [i]() { return i + "b"; }; };
+//   auto next2 = [](std::string i) { return [i]() { return i + "c"; }; };
+//   EXPECT_EQ(Seq(initial, next1, next2)(), Poll<std::string>("abc"));
+// }
+//
+// For a complete understanding of all possible uses and nuances of Seq look at
+// ThreeTypedPendingThens in file seq_test.cc
+
 namespace promise_detail {
 
 template <typename T>
@@ -76,12 +117,6 @@ using SeqIter = BasicSeqIter<SeqTraits, Iter, Factory, Argument>;
 
 }  // namespace promise_detail
 
-// Sequencing combinator.
-// Run the first promise.
-// Pass its result to the second, and run the returned promise.
-// Pass its result to the third, and run the returned promise.
-// etc
-// Return the final value.
 template <typename F>
 GPR_ATTRIBUTE_ALWAYS_INLINE_FUNCTION inline F Seq(F functor) {
   return functor;