doc: drivers: system timer: unique documentation header

Without enough context, the "clock" term is underspecified in Zephyr.
This change renames "Clock API" to "System Clock API" as both the
functions as well as the remaining documentation calls the clock being
documented here this way.

This tries to ensure that people landing on this API from context free
search will be less confused.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>