docs: update SelectiveGapicGeneration usage doc #105964

Job	Run time
CI (macos-latest, 4.0, --test)	2m 18s
CI (ubuntu-latest, 3.3, --test)	3m 1s
CI (ubuntu-latest, 3.4, --test)	2m 47s
CI (windows-latest, 4.0, --test)	6m 27s
CI (ubuntu-latest, 3.2, --test)	2m 45s
CI (ubuntu-latest, 3.1, --test)	41s
CI (ubuntu-latest, 4.0, --test)	2m 29s
CI (ubuntu-latest, 4.0, --rubocop-toplevel --rubocop --build --yard --linkinator)	6m 34s
	27m 2s

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