Fix doc of AVR-specific command "info io_registers"

Fix doc of AVR-specific command "info io_registers"

Running the selftests on an all-targets build, I get:

    Running selftest help_doc_invariants.
    help doc broken invariant: command 'info io_registers' help doc first line is not terminated with a '.' character
    Self test failed: self-test failed at /home/simark/src/binutils-gdb/gdb/unittests/help-doc-selftests.c:95

Add a period at the end of the doc of that command, and make it a bit
nicer in general.

gdb/ChangeLog:

	* avr-tdep.c (_initialize_avr_tdep): Improve help of command
	"info io_registers".