releases/15.0R/relnotes: Move back asinf(3)/acos(3) under libraries

The "The accuracy of asinf(3) and acosf(3) has improved." paragraph
inadvertently slipped into the "Deprecated Applications" sub-section.
Put it back under "Runtime Libraries and API".

These notes were copied from those of 14.2, and the mixup occured
because of commit f4f1e3f1e4 ("14.2/relnotes: Add some more entries")
and the one referenced by "Fixes:" below.

Fixes: f232709d21 ("14.2/relnotes: fix manpage for acosf(3) and add Oracle Cloud URL")
Sponsored by: The FreeBSD Foundation