Improve Python Unwinders documentation

- avoid "GDB proper" to refer to global locus, as object files and
  program spaces are also GDB proper.

- gdb.register_unwinder does not accept locus=gdb.

- "a unwinder" -> "an unwinder"

Approved-by: Eli Zaretskii <eliz@gnu.org>
Change-Id: I98c1b1000e1063815238e945ca71ec6f37b5702e