docs(auth): clarify quota project behavior on ImpersonatedCredentials

[lqiu96]

Fixes #12604

Summary

Clarifies the Javadoc on ImpersonatedCredentials (Builder.setQuotaProjectId, idTokenWithAudience, and create overloads) regarding how quotaProjectId is applied:

• The quotaProjectId configured on ImpersonatedCredentials is sent via the x-goog-user-project header on downstream API requests authenticated with the impersonated credentials.
• Requests to the IAM Credentials API (generateAccessToken and generateIdToken) used to mint tokens are authenticated using sourceCredentials. To specify a quota project (x-goog-user-project header) for those IAM Credentials API calls, the quota project ID should be configured on sourceCredentials.

[gemini-code-assist]

Code Review

This pull request updates the Javadoc documentation in ImpersonatedCredentials.java to clarify the behavior of the quotaProjectId parameter and builder method. Specifically, it explains that the quota project ID is applied to downstream requests made with the impersonated credentials, but not to the IAM Credentials API requests (such as generateAccessToken or generateIdToken) used to mint tokens. To set a quota project for those token-minting calls, users must configure it on the sourceCredentials instead. Additionally, a minor typo in the documentation was corrected. There are no review comments, and I have no further feedback to provide.