PHPDoc comments and types missing in Translation_Entry

[pedromendonca]

Function and property declarations are missing PHPDoc comment, types declaration and @return tags at many places of src/wp-includes/pomo/entry.php.

Proposed patch added.

Add missing Docblocks and declare types of Translation_Entry properties.

Trac ticket: 65901

## Use of AI Tools

AI assistance: Yes
Tool: Claude
Model: Opus 5
Used for: Initial assessment and code skeleton; final implementation was reviewed and edited by me.

## Trac Ticket
https://core.trac.wordpress.org/ticket/65901

## Description

Adds missing PHPDoc blocks and type declarations to the Translation_Entry class in src/wp-includes/pomo/entry.php.

### Changes

Property docblocks added for all previously undocumented properties:

• $singular — string|null
• $plural — string|null
• $translations — (string|null)[]
• $context — string|null
• $translator_comments — string
• $extracted_comments — string
• $references — string[]
• $flags — string[]

Constructor docblock improved:

• Added @since 2.8.0 tag
• Added summary line (Constructor.)
• Updated parameter description to Optional. Array of arguments. Default empty array.
• Updated @type annotations with precise types (string[] instead of array, (string|null)[] for translations)

Other improvements:

• Added missing @param tag to the deprecated PHP4 constructor Translation_Entry()
• Added @return void to merge_with()
• Fixed period at end of file-level docblock description

Fixes #65901.

Duplicate of #13126.

[SergeyBiryukov]

In 63323:

Docs: Add missing descriptions and declare types for Translation_Entry properties.

Developed in ​https://github.com/WordPress/wordpress-develop/pull/13126.

Follow-up to r10584, r32990, r57734.

Props pedromendonca, mukesh27, dhruvang21.
Fixes #65901.