Documentation: Fill the gaps about entry/noinstr constraints

The entry/exit handling for exceptions, interrupts, syscalls and KVM is
not really documented except for some comments.

Fill the gaps.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Co-developed-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>

----

Changes since v3:
 - s/nointr/noinstr/

Changes since v2:
 - No big content changes, just style corrections, so it should be
   pretty clean at this stage. In the light of this, I kept Mark's
   Reviewed-by.
 - Paul's style and paragraph re-writes
 - Randy's style comments
 - Add links to transition type sections

Documentation/core-api/entry.rst | 261 +++++++++++++++++++++++++++++++
 Documentation/core-api/index.rst |   8 +
 2 files changed, 269 insertions(+)
 create mode 100644 Documentation/core-api/entry.rst

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20220110105044.94423-1-nsaenzju@redhat.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>