`status_internal`: add note about potential status message optimization

In short, if most statuses are constructed with empty messages, or messages that are too long to fit into `std::string`'s local storage, we might save memory by getting rid of the local storage and always putting messages on the heap.

PiperOrigin-RevId: 949730457
Change-Id: I7f47f036a16677c926078c7d48dabf8166af0f55

absl/status/internal/status_internal.h

diff --git a/absl/status/internal/status_internal.h b/absl/status/internal/status_internal.h
index 59ce36a..767f88f 100644
--- a/absl/status/internal/status_internal.h
+++ b/absl/status/internal/status_internal.h
@@ -150,7 +150,13 @@
   // As an internal implementation detail, we guarantee that if status.message()
   // is non-empty, then the resulting string_view is null terminated.
   // This is required to implement 'StatusMessageAsCStr(...)'
+  //
+  // NOTE: if most statuses are constructed with messages that are either empty
+  // or so long they don't fit in the std::string's local storage (small string
+  // optimization), replacing std::string with an entirely heap-allocated
+  // string might save memory at scale.
   std::string message_;
+
   absl::InlinedVector<absl::SourceLocation, 1> source_locations_;
   std::unique_ptr<status_internal::Payloads> payloads_;
 };