gdb: change bpstat_print's kind parameter to target_waitkind

Change from int to target_waitkind,  which is really what is is.  While
at it, remove some outdated doc.  The return value is described by a
relatively self-describing enum, not a numerical value like the doc
says.

Change-Id: Id899c853a857c7891c45e5b1639024067d5b59cd