kcov: improve documentation

Improve KCOV documentation:

- Use KCOV instead of kcov, as the former is more widely-used.

- Mention Clang in compiler requirements.

- Use ``annotations`` for inline code.

- Rework remote coverage collection documentation for better clarity.

- Various smaller changes.

[andreyknvl@google.com: v2]
  Link: https://lkml.kernel.org/r/583f41c49eef15210fa813e8229730d11427efa7.1677614637.git.andreyknvl@google.com
[andreyknvl@google.com: fix ``annotation`` for KCOV_REMOTE_ENABLE]
  Link: https://lkml.kernel.org/r/72be5c215c275f35891229b90622ed859f196a46.1677684837.git.andreyknvl@google.com
Link: https://lkml.kernel.org/r/0b5efd70e31bba7912cf9a6c951f0e76a8df27df.1677517724.git.andreyknvl@google.comSigned-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>