Add documentation on how clearing Python proto fields does not reclaim memory.

Add documentation on how clearing Python proto fields does not reclaim memory. For those unfamiliar with this behavior, debugging memory creep as a result of it can be time consuming. Adding some documentation to help them more easily find the issue.

PiperOrigin-RevId: 888653180

Author: protobuf-github-bot

content/reference/python/python-generated.md

@@ -723,6 +723,31 @@ possible message creation more explicit:
 m.message_map.get_or_create(10)
 ```
 
+### Memory management {#clearing-fields}
+
+Note that clearing a field (via `ClearField`, `Clear`, `del` (for lists) or
+`ClearExtension`) doesn't necessarily reclaim memory. It just resets the object
+to look as if things are unset, but the subobjects *may* continue to exist. For
+example, clearing a repeated field sets its size to 0, but does not relinquish
+its capacity (analogous to calling `.clear()` on a `std::vector` in C++).
+
+There are various ways in which to ensure the memory is freed up. One method is
+to make a copy of the proto after clearing the fields, as the unused memory is
+not copied over:
+
+```python
+# Assuming 'my_proto' has a memory-intensive field 'big_field'
+# Clears the field. Memory might still be reserved.
+my_proto.ClearField('big_field')
+
+# Copy to a new instance. This allocates only necessary memory.
+compact_proto = my_proto_pb2.MyProto()
+compact_proto.CopyFrom(my_proto)
+
+# Once all references to the initial proto are gone, it is garbage collected.
+my_proto = compact_proto
+```
+
 ## Enumerations {#enum}
 
 In Python, enums are just integers. A set of integral constants are defined