Docs: improvements for "references - phpdoc - tags - license"
Docs: improvements for "references - phpdoc - tags - license"

Description:
* Rephrased the "NOT RECOMMENDED" phrase to use "RECOMMENDED ... ONLY" for clarity.

Effects in phpDocumentor:
* Minor text clarification.

Example:
* Added the PHP open tag to reinforce the recommendation to only use the tag in a file-level PHPDoc.
* Added a blank comment line to make it clearer that these are alternatives and not tags to be used together in the same PHPDoc.

Other:
* Minor inline markup and grammar fixes.
* Made the syntax outline a code block.

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