Document sgtl5000

Remove blah blah. Document based on control_sgtl500.h and
control_sgtl500.cpp.
Clarify that only one frequency modifier can be active at one.  Add
suggestions for pop-free ramping as indicated in datasheet.
Re-order function descriptions to group related ones and put in order of
signal flow.

Add #defines to control_sgtl5000.h to make selecting frequency modifier
(such as tone control) easier.