kernel: irq: update irq_lock() documentation regarding context switches

Upon context switch, the virtual "interrupt lock" acquired by irq_lock()
must not be "held". However, the current documentation for irq_lock() says
that it is perfectly valid to hold it (!), and that a suspended thread will
hold the "interrupt lock" upon being scheduled again (!!).

Update the documentation to remove the outdated section and indicate that
context switching while holding the interrupt lock is not allowed.

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