WebView: update SafeBrowsing documentation

No change in logic, only docs changes:

 * Docs about Safe Browsing feature as a whole.
 * Expand the docs for initSafeBrowsing/shutdownSafeBrowsing

Bug: 63660204
Bug: 62192626
Test: make update-api and manually check javadocs
Change-Id: I6526a0f1ce1327f6eff416c27ded89866587f173