tracing: Add documentation for funcgraph-retval and funcgraph-retval-hex

Add documentation for the two newly introduced options for the
function_graph tracer. The funcgraph-retval option is used to
control whether or not to display the return value, while the
funcgraph-retval-hex option is used to control the display
format of the return value.

Link: https://lkml.kernel.org/r/2b5635f05146161b54c9ea6307e25efe5ccebdad.1680954589.git.pengdonglin@sangfor.com.cn

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Donglin Peng <pengdonglin@sangfor.com.cn>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>