Edit the section "bluefs-bdev-migrate" in
doc/man/8/ceph-bluestore-tool.rst to add the information that this
operation expands the target storage by updating its size label, making
"bluefs-bdev-expand" unnecessary.
Improve the subject-verb agreement in this section, and supply some
absent definite articles.
Co-authored-by: Peter Gervai <grin@drop.grin.hu> Signed-off-by: Zac Dover <zac.dover@proton.me>