vdpa: fix grammar, titles, and formatting in man pages

This patch addresses grammatical errors, copy-paste errors from devlink
documentation, and formatting issues across the vdpa man page documentation.

Changes by file:

vdpa.8:
  - Remove unnecessary article: "a pretty JSON output" -> "pretty JSON output"

vdpa-dev.8:
  - Fix incorrect title: "DEVLINK\-DEV" -> "VDPA\-DEV"
  - Fix file path: "virtio*h" -> "virtio*.h" (add missing period)
  - Fix capitalization and punctuation in macro description
  - Split overly long vstats section header into title + body
  - Fix spacing: "-Name" -> "- Name"
  - Fix article error: "the all vdpa devices" -> "all vdpa devices"
  - Add missing periods to example descriptions (2 instances)

vdpa-mgmtdev.8:
  - Fix incorrect title: "DEVLINK\-DEV" -> "VDPA\-MGMTDEV"
  - Fix NAME section: "vdpa-dev" -> "vdpa-mgmtdev"

The title corrections in vdpa-dev.8 and vdpa-mgmtdev.8 fix critical
copy-paste errors that would cause man page indexing problems and
confusion about which command is being documented.

No functional changes. All technical content remains accurate.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David Ahern <dsahern@kernel.org>