WebView: Clarify documentation about mimeTypes

This change edits the WebView documentation to specify that the mimeType
parameter to loadData and loadDataWithBaseUrl should be a valid mimeType
and the data will be downloaded if it is not.

Test: manual - this only changes documentation
Bug: 70528830
Change-Id: I58ca4655673c6ccfe7547572579ef86a14a259ed