doc: kernel: fatal: update "Build Assertions" section intro paragraph

A long time ago, Zephyr had two macros that could be used for build-time
assertions: BUILD_ASSERT() and BUILD_ASSERT_MSG(). The latter has been
dropped in v2.7 and removed from the documentation; however, the intro
paragraph of the "Build Assertions" section has never been updated to
reflect this, and still confusingly claims that "Zephyr provides two
macros for performing build-time assertions" when only BUILD_ASSERT()
remains.

Update the introductory paragraph of "Build Assertions" section such that
it makes sense now that only one build-time assertion macro exists.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>