add documentation for python.DisableFieldCache annotation

Summary: Improve explanation of `python.DisableFieldCache`

Reviewed By: prakashgayasen

Differential Revision: D78364087

fbshipit-source-id: d31650ce83a483a90846901f1538326e141f87fb

Author: ahilger

thrift/annotation/python.thrift

@@ -128,5 +128,15 @@ struct DeprecatedSortSetOnSerialize {}
 struct DeprecatedKeySortMapOnSerialize {}
 
 /// Disable caching all fields for a struct.
+/// Also available as a thrift_library compiler option:
+///     thrift_library(..., thrift_python_options = ["disable_field_cache"])
+///
+/// Has NO effect in cinder runtime (e.g., IG Django)
+/// Only affects thrift-python, not older deprecated variants.
+///
+/// Usage guidelines:
+///   - Improves latency/throughput when struct fields accessed only once.
+///   - Reduces memory usage for most use cases; try this to resolve OOMs.
+///   - Worsens latency for subsequent field accesses relative to default.
 @scope.Struct
 struct DisableFieldCache {}