Update comments about removed function

Update comments about removed function

regset_from_core_section doesn't exist anymore; it has been replaced
by the iterate_over_regset_sections gdbarch method.  Update comments
accordingly to not confuse readers.

gdb/ChangeLog:

2020-01-24  Christian Biesinger  <cbiesinger@google.com>

	* aarch64-fbsd-tdep.c (aarch64_fbsd_iterate_over_regset_sections):
	Update comment.
	* aarch64-linux-tdep.c (aarch64_linux_iterate_over_regset_sections):
	Likewise.
	* arm-fbsd-tdep.c (arm_fbsd_iterate_over_regset_sections): Likewise.
	* gdbcore.h (deprecated_add_core_fns): Update comment to point to
	the correct replacement (iterate_over_regset_sections).
	* riscv-fbsd-tdep.c (riscv_fbsd_iterate_over_regset_sections):
	Update comment.

Change-Id: I5eea4d18e15edae5d6dfd5d0d6241e5b2ae40daa