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

Description:
* Removed a duplicate sentence.

Other:
* Minor inline markup and grammar fixes.
* Made the syntax outline a code block.
* Linked some text to related other documentation pages.
* Linked some text to the PHP manual.

phpDocumentor/phpDocumentor2,phpDocumentor/phpDocumentor2,phpDocumentor/phpDocumentor2,phpDocumentor/phpDocumentor2