docs: update SelectiveGapicGeneration usage doc #101812

Job	Run time
CI (ubuntu-latest, 3.2, --test)	3m 32s
CI (ubuntu-latest, 4.0, --rubocop-toplevel --rubocop --build --yard --linkinator)	23m 21s
CI (ubuntu-latest, 3.1, --test)	54s
CI (windows-latest, 4.0, --test)	7m 34s
CI (ubuntu-latest, 3.3, --test)	3m 21s
CI (macos-latest, 4.0, --test)	2m 56s
CI (ubuntu-latest, 3.4, --test)	3m 26s
CI (ubuntu-latest, 4.0, --test)	3m 34s
	48m 38s

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