Add comment to document secondary role of QuicSpdySession::qpack_maximum_dynamic_table_capacity_.

This is a follow-up to
https://quiche.googlesource.com/quiche/+/40e9a7ba152f774b29218f5ff5321f7519df8c6d.

gfe-relnote: n/a, change to QUIC v99-only code.  Protected by existing disabled gfe2_reloadable_flag_quic_enable_version_99.
PiperOrigin-RevId: 268770481
Change-Id: Ib420571fb5e83b7e804a29570e8f955cde666dfe

quic/core/http/quic_spdy_session.h

diff --git a/quic/core/http/quic_spdy_session.h b/quic/core/http/quic_spdy_session.h
index 0ee3632..faba099 100644
--- a/quic/core/http/quic_spdy_session.h
+++ b/quic/core/http/quic_spdy_session.h
@@ -321,6 +321,9 @@
   // https://quicwg.org/base-drafts/draft-ietf-quic-qpack.html#maximum-dynamic-table-capacity
   // for the decoding context.  Value will be sent via
   // SETTINGS_QPACK_MAX_TABLE_CAPACITY.
+  // |qpack_maximum_dynamic_table_capacity_| also serves as an upper bound for
+  // the dynamic table capacity of the encoding context, to limit memory usage
+  // if a larger SETTINGS_QPACK_MAX_TABLE_CAPACITY value is received.
   uint64_t qpack_maximum_dynamic_table_capacity_;
 
   // Maximum number of blocked streams as defined at