chore: Improve backend in-code documentation for managing default accounts

[aterga]

Motivation

This PR addresses follow-up from #3374

Changes

• Updated comments to use "synthetic account" instead of "default account" terminology
• Enhanced function documentation with clearer explanations of behavior and error conditions
• Fixed a spelling error in documentation

Tests

No new tests needed.

[Copilot]

Pull Request Overview

This PR improves in-code documentation by updating terminology from "default account" to "synthetic account" for clarity and consistency across the codebase. The changes focus on making the documentation more precise about the nature of accounts that are not meant to be stored.

• Updated comments to use "synthetic account" instead of "default account" terminology
• Enhanced function documentation with clearer explanations of behavior and error conditions
• Fixed a spelling error in documentation

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/internet_identity_interface/src/internet_identity/types.rs	Updated comment to use "synthetic account" terminology
src/internet_identity/src/storage/storable/anchor_application_config.rs	Updated comment to use "synthetic account" terminology
src/internet_identity/src/storage/storable/account_reference.rs	Updated comment to use "synthetic account" terminology
src/internet_identity/src/storage/account.rs	Enhanced documentation for synthetic account concept with additional context
src/internet_identity/src/account_management.rs	Improved function documentation with detailed behavior explanations and fixed spelling error

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[lmuntaner]

Thanks!