python: Fix erroneous doc about gdb.objfiles()

python: Fix erroneous doc about gdb.objfiles()

The code implementing gdb.objfiles() returns a list of objfiles for the
current program space (the program space of the selected inferior).  The
documentation for the gdb.objfiles() Python method, however, states:

    Return a sequence of all the objfiles current known to GDB.

That sounds wrong to me.  I tried to phrase to be more precise.

gdb/doc/ChangeLog:

	* python.texi (Objfiles In Python): Update gdb.objfiles() doc.