docs: improve docstring for get_serializer to clarify behavior

[MoeSalah1999]

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Description

Summary

This PR adds missing docstrings to two internal helper functions in serializers.py that are central to serializer validation and error handling.

The changes are documentation-only and do not alter runtime behavior.

Details

The following docstrings were added:

as_serializer_error

Documents the function’s role in normalizing Django and DRF ValidationError instances into the serializer .errors format.

Clarifies the expected structure of field and non-field errors returned by the function.

raise_errors_on_nested_writes

Replaces inline explanatory comments with a formal docstring.

Explains why writable nested and dotted-source fields are intentionally unsupported by default.

Documents when the helper is invoked and what is expected of developers when overriding create() / update().

These helpers are frequently encountered when extending or debugging serializers, but previously lacked API-level documentation.

Motivation

Serializer error handling and nested write behavior are common sources of confusion for contributors and advanced users. Providing clear docstrings improves:

Code readability and maintainability

IDE introspection and contributor onboarding

Alignment between implementation and documented behavior

Scope

Documentation-only change

No functional or behavioral modifications

No test changes required

Checklist

• Documentation-only change
• No behavior changes
• Code style follows existing conventions