Clarify WebResourceResponse parameter documentation.

Developers commonly misuse the mimeType and encoding parameters in the
WebResourceResponse constructor as the meaning is different to the
similarly named HTTP headers. Explain more carefully what valid/invalid
inputs look like and mention that this is different to HTTP.

Change-Id: I14d159bc50c89c2215f7de5f4732569e5e246930
Fixes: 64765685
Test: m offline-sdk-docs