Explain sscratch is only swapped by software (#1722)

* Explain `sscratch` is only swapped by software

Someone familiar with the interrupt handlers of other ISAs may
incorrectly imagine that the current description means that RISCV cores
will automatically perform the swap of `sscratch` with a user register.

This is not the case: it is a recommendation disguised as a description.
Flesh it out a bit so it is less ambiguous.

* Tweak sscratch description

Signed-off-by: Andrew Waterman <aswaterman@gmail.com>

---------

Signed-off-by: Andrew Waterman <aswaterman@gmail.com>
Co-authored-by: Andrew Waterman <aswaterman@gmail.com>

src/supervisor.adoc

@@ -490,8 +490,10 @@ access a counter if the corresponding bits in `scounteren` and
 The `sscratch` CSR is an SXLEN-bit read/write register, dedicated
 for use by the supervisor. Typically, `sscratch` is used to hold a
 pointer to the hart-local supervisor context while the hart is executing
-user code. At the beginning of a trap handler, `sscratch` is swapped
-with a user register to provide an initial working register.
+user code.
+At the beginning of a trap handler, software normally uses a CSRRW
+instruction to swap `sscratch` with an integer register to obtain an
+initial working register.
 
 .Supervisor Scratch Register
 include::images/bytefield/sscratch.edn[]