WebView: document Safe Browsing and shouldInterceptRequest

No change to logic, docs only.

This documents shouldInterceptRequest request behavior when Safe
Browsing is enabled, with recommendations for how this can be avoided
depending on the application's needs.

Bug: 65555402
Test: make docs (manually verify links all work)
Change-Id: I055254bfae3a06061402c519e8740ec4d9779e8f