[PATCH] Document tracepoint restrictions

[Stan Shebs]

Eli Zaretskii wrote:
>> + bytes from the stack pointer with something like @code{*$esp@@300}
>> + (adjust to use the name of the actual stack pointer register on your
>> + target architecture, and the amount of stack you wish to capture).
>>     
>
> Isn't it better to just use the generic $sp here?  Or are you saying
> that it is not supported by tracepoints?
>   
$sp is a pseudo-register, and garners a complaint right now.
> Okay with these changes.
>   
Thanks, I'll make the fixes!

Stan