Improve documentation for core is-installed command (#237) #334

Job	Run time
test / Functional - WP latest on PHP 5.6 with MySQL 8.0	6m 4s
test / Functional - WP latest on PHP 7.0 with MySQL 8.0	6m 47s
test / Functional - WP latest on PHP 7.1 with MySQL 8.0	5m 2s
test / Functional - WP latest on PHP 7.2 with MySQL 8.0	6m 17s
test / Functional - WP latest on PHP 7.3 with MySQL 8.0	5m 59s
test / Functional - WP latest on PHP 7.4 with MySQL 8.0	4m 45s
test / Functional - WP latest on PHP 8.0 with MySQL 8.0	5m 7s
test / Functional - WP latest on PHP 8.1 with MySQL 8.0	5m 33s
test / Functional - WP latest on PHP 8.2 with MySQL 8.0	4m 22s
test / Functional - WP trunk on PHP 5.6 with MySQL 8.0	4m 0s
test / Functional - WP trunk on PHP 5.6 with MySQL 5.7	5m 2s
test / Functional - WP trunk on PHP 5.6 with MySQL 5.6	3m 7s
test / Functional - WP trunk on PHP 7.4 with MySQL 8.0	6m 41s
test / Functional - WP trunk on PHP 8.0 with MySQL 8.0	5m 41s
test / Functional - WP trunk on PHP 8.0 with MySQL 5.7	5m 9s
test / Functional - WP trunk on PHP 8.0 with MySQL 5.6	5m 49s
test / Functional - WP trunk on PHP 8.1 with MySQL 8.0	6m 1s
test / Functional - WP trunk on PHP 8.2 with MySQL 8.0	5m 10s
test / Functional - WP 3.7 on PHP 5.6 with MySQL 5.6	5m 15s
test / Unit test / PHP 5.6	5s
test / Unit test / PHP 7.0	4s
test / Unit test / PHP 7.1	6s
test / Unit test / PHP 7.2	6s
test / Unit test / PHP 7.3	6s
test / Unit test / PHP 7.4	4s
test / Unit test / PHP 8.0	5s
test / Unit test / PHP 8.1	5s
test / Unit test / PHP 8.2	7s
	1h 42m 39s

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