Add a description of the similar packets defined in the

gdb-remote serial protocol documentation, call out the 
incompatability of lldb's vFile:open: packet as it stands
today.  Need to think about whether to change lldb's
enum values (breaking any existing lldb-server's out there)
or create a different packet and abandon vFile:open: at
least for a while.

llvm-svn: 349316