doc: Improve the signing docs #923

Job	Run time
AutoReview Tests	14s
Wait for the PHAR & Docker images	4m 37s
Lint CS	22s
Unit Tests (PHP 8.1, locked)	24s
Unit Tests (PHP 8.1, lowest)	19s
Unit Tests (PHP 8.1, highest)	23s
Update the RequirementChecker	0s
Unit Tests (PHP 8.2, highest)	20s
Unit Tests (PHP 8.3, highest, --ignore-platform-req=php+)	1m 10s
End-to-End Tests (_test_e2e_pass_no_config_min_composer_php, ghcr.io/box-project/box_php725)	17s
End-to-End Tests (_test_e2e_pass_no_config_min_box_php, ghcr.io/box-project/box_php81)	34s
End-to-End Tests (_test_e2e_pass_complete_min_composer_php, ghcr.io/box-project/box_php725)	19s
End-to-End Tests (_test_e2e_pass_complete_min_box_php, ghcr.io/box-project/box_php81)	29s
End-to-End Tests (_test_e2e_fail_complete_min_composer_php, ghcr.io/box-project/box_php725)	18s
End-to-End Tests (_test_e2e_fail_complete_min_box_php, ghcr.io/box-project/box_php81)	31s
End-to-End Tests (_test_e2e_skip_min_composer_php, ghcr.io/box-project/box_php725)	17s
End-to-End Tests (_test_e2e_pass_complete_requirement_checker_not_disabled, ghcr.io/box-project/b...	28s
End-to-End Tests (_test_e2e_fail_conflict, ghcr.io/box-project/box_php81)	32s
RequirementChecker status	2s
	11m 36s

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