[lldb][docs] Document SBType

This documents the behaviour of the different SBType functions with notes for
the language-specific behaviour for C/C++/Objective-C. All of this reflects the
current behaviour of LLDB (even though that also means some functions behave
kinda weird but at least they are now documented to be weird)

Reviewed By: #lldb, mib

Differential Revision: https://reviews.llvm.org/D103454