aio_mlock.2: Expand document description

[ziaee]

Summary

+ The old one renders broken because Xr is not correct/portable in Nd
+ Take this opportunity to explain it better using mlock description

MFC after: 3 days

[glebius]

Thanks! I don't think such trivial changes need review process.

[ziaee]

In D61334#1409036, @glebius wrote:

Thanks! I don't think such trivial changes need review process.

Thanks for looking it over, the reason for review is that I am not the target audience for syscalls and might have misunderstood, or maybe you would want something else.