docs: update description on second-instance event. (#23119)
docs: update description on second-instance event. (#23119)

* Update app.md

more descriptions on second-instance event.

* Update docs/api/app.md

Co-Authored-By: loc <e3579b1e47f273529f0f929453e939a68ede9fd1@slack-corp.com>

Co-authored-by: Cheng Zhao <bedccee303893728210df61409d3dc544e3be3a6@electronjs.org>
Co-authored-by: loc <e3579b1e47f273529f0f929453e939a68ede9fd1@slack-corp.com>
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