Improve documentation of ScanResult and WifiInfo.

Valid dBm ranges should *not* be mentioned in the documentation.
Manufacturers should be able to produce devices which can detect
signal strengths with arbitrary sensitivity rather than being forced
to restrict strengths to a certain range.

Adds a reference to WifiManager.calculateSignalLevel(), an
often-useful method to convert these RSSI values into an absolute
signal level that can be displayed to a user as the signal strength.

Bug: 13731529
Change-Id: I0aae85a6d8bc48ffddd1927d816a1dfbf5230f7e