book: Add information about libloading #2791

Job	Run time
docs	26s
check-cfg	21s
minimal	19s
msrv	45s
rustfmt-clippy	34s
quickchecking	23s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 46s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	1m 45s
test-book	7s
test-no-headers	24s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	1m 45s
test-expectations (ubuntu-latest)	31s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	1m 53s
test-expectations (macos-latest)	28s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	2m 22s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	2m 15s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	2m 15s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	2m 20s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	4m 21s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	4m 21s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	4m 14s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	4m 16s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	5m 4s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	5m 55s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	4m 50s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	4m 45s
test (ubuntu-latest, 16.0, 0, 0, 1)	2m 31s
test (macos-latest, 16.0, 0, 0, 0)	2m 19s
success	1s
	1h 3m 16s

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