docs: clarify getGenericPassword false result

[wanxiankai]

Summary

Fixes #785 by clarifying what getGenericPassword() returning false means.

• Updates the getGenericPassword() TSDoc return description to say false means no credentials exist for the requested service
• Clarifies that Keychain access errors reject the promise instead of being represented by false
• Adds the same clarification to the basic usage example

Verification

• git diff --check
• rg -n 'false.*failure|getGenericPassword.*false|no credentials exist|Keychain access errors|false\ means no credentials' src/index.ts website/docs/usage.md`

Note: this is a documentation/TSDoc-only change. I did not run full lint/typecheck because dependencies are not installed in the local checkout.