[PHPDoc] Improved ContentService doc for API reference #2357

Job	Run time
Unit tests & SQLite integration tests (7.4)	9m 11s
Run code style check (8.0)	2m 38s
Unit tests & SQLite integration tests (8.0)	8m 59s
Solr integration tests (7.4)	13m 15s
Unit tests & SQLite integration tests (8.1)	8m 53s
Solr integration tests (8.0)	13m 21s
Solr integration tests (8.1)	13m 9s
PostgreSQL integration tests (7.4)	28m 52s
PostgreSQL integration tests (8.0)	28m 34s
PostgreSQL integration tests (8.1)	27m 10s
MySQL integration tests (7.4)	15m 0s
MySQL integration tests (8.0)	14m 43s
MySQL integration tests (8.1)	14m 28s
	3h 18m 13s

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