Document DAP 'launch' parameter

Document DAP 'launch' parameter

The Debugger Adapter Protocol defines a "launch" request but leaves
the parameters up to the implementation:

    Since launching is debugger/runtime specific, the arguments for
    this request are not part of this specification.

This patch adds some documentation for the parameter GDB currently
defines.  Note that I plan to add more parameters here, and perhaps
there will be other extensions in time as well.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>