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>
1 file changed