Documentation: Call out example SYM_FUNC_* usage as x86-specific

Documentation: Call out example SYM_FUNC_* usage as x86-specific

The example given in asm-annotations.rst to describe the constraints that
a function should meet in order to be annotated with a SYM_FUNC_* macro
is x86-specific, and not necessarily applicable to architectures using
branch-and-link style calling conventions such as arm64.

Tweak the example text to call out the x86-specific text.

Cc: Mark Brown <broonie@kernel.org>
Cc: Jiri Slaby <jslaby@suse.com>
Signed-off-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200115184305.1187-1-will@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>