arrows: Refresh HdyArrowsDirection docs

This moves the HdyArrowsDirection documentation to the C file and
removes the final period from the values definitions, like for all other
enums documentations.
5 jobs for arrows-direction-doc in 3 minutes and 46 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #18864
librem5
build-debian-clang

00:02:30

passed #18862
librem5
build-debian-gcc

00:03:01

37.8%
passed #18863
librem5
build-debian-gcc-static

00:01:59

passed #18866
librem5
build-debian-package

00:01:46

passed #18865
librem5
build-fedora-gcc

00:02:47