dcb: fix grammar and style issues in man pages

This patch addresses various grammatical errors, style inconsistencies,
and clarity issues across the DCB man page documentation.

Changes by file:

dcb.8:
  - Capitalize first word of -n option description
  - Add missing article "The" in -b option description
  - Fix verb form "shown" -> "show" in -s option description
  - Add missing dcb-dcbx(8) reference in SEE ALSO section

dcb-app.8:
  - Add missing article "the" before "APP table"
  - Fix singular/plural: "value" -> "values"
  - Improve phrasing: "priority of both 1 and 2" -> "priorities 1 and 2"
  - Make command descriptions consistent (present tense)
  - Simplify relative clause construction in default-prio description
  - Fix PCP/DEI description formatting and punctuation

dcb-apptrust.8:
  - Remove inappropriate period in NAME section
  - Improve clarity by removing unnecessary commas and redundant phrasing
  - Use present tense instead of future in DESCRIPTION
  - Make command descriptions consistent with proper articles

dcb-buffer.8:
  - Remove unnecessary "be" in DESCRIPTION
  - Add missing comma in prio-buffer parameter description

dcb-dcbx.8:
  - Capitalize first word of static parameter description

dcb-ets.8:
  - Add missing commas after introductory phrases in prio-tc and
    tc-tsa parameter descriptions

dcb-rewr.8:
  - Capitalize month name in TH date
  - Fix subject-verb agreement and plural forms in DESCRIPTION
  - Make command descriptions consistent (present tense)
  - Fix PCP/DEI description formatting and punctuation
  - Add missing commas in SEE ALSO section
  - Fix spacing in REPORTING BUGS section

No functional changes. All technical content remains accurate.

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