Documentation: kbuild: Improve grammar

  - singular versus plural,
  - "by" versus "of",
  - missing "if", "it", "the",
  - consistent use of "xxx-specific" versus "xxx specific".

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>