docs:core-api: fixed typos and grammar in printk-index page

Signed-off-by: Dennis Lam <dennis.lamerice@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240502212522.4263-1-dennis.lamerice@gmail.com

printk/index: Printk index feature documentation

Document the printk index feature. The primary motivation is to
explain that it is not creating KABI from particular printk() calls.

Acked-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Chris Down <chris@chrisdown.name>
Signed-off-by: Petr Mladek <pmladek@suse.com>