[lldb] Document the SBDebugger public interface

Despite our best efforts, improved documentation of the public API
remains a common request amongst its users. We have the Docstrings files
in the bindings directory, but coverage and quality varies greatly by
class.

I've noticed that we a bunch of Doxygen comments throughout LLDB, but
relatively few in the SB API, which might be explained by the
aforementioned Docstrings files. With llvm#147617, we can leverage the
Doxygen documentation to automatically generate Pydoc documentation for
the Python bindings using SWIG.

Going forward, I'd like to invest in the Doxygen documentation for the
public API. This PR is a first step in that direction. The Doxygen
comments will benefit both C++ and Python clients. Additionally, the
compiler (through the -Wdocumentation flag) can help catch quality and
correctness issues.

Author: JDevlieghere