Improve doc blocks for JMenu

[mbabker]

Summary of Changes

Improve doc blocks to highlight the returned JMenuItem object from the JMenu API.

Testing Instructions

Review

Documentation Changes Required

This is the documentation.

[jeckodevelopment]

I have tested this item ✅ successfully on e19fb2b

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/13274.}

[gwsdesk]

Review shows no issues so works4me ✅

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/13274.}

[csthomas]

I have tested this item ✅ successfully on e19fb2b

Code review.

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/13274.}

[jeckodevelopment]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/13274.}