docs/emulation: expand warning about semihosting

A surprising feature of calls like SYS_READC is this can cause QEMU to
indefinitely block as there is no handling for EOF.

Clarifies: https://gitlab.com/qemu-project/qemu/-/issues/1963
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231120150833.2552739-8-alex.bennee@linaro.org>

docs/about/emulation.rst

@@ -129,8 +129,9 @@ causing most hypervisors to trap and fault on them.
 .. warning::
    Semihosting inherently bypasses any isolation there may be between
    the guest and the host. As a result a program using semihosting can
-   happily trash your host system. You should only ever run trusted
-   code with semihosting enabled.
+   happily trash your host system. Some semihosting calls (e.g.
+   ``SYS_READC``) can block execution indefinitely. You should only
+   ever run trusted code with semihosting enabled.
 
 Redirection
 ~~~~~~~~~~~