Shouldn't we expand contractions?

uname(8) -> .Xr uname 1

$ man 8 uname
No manual entry for uname

sh(1) -> .Xr sh 1

login(1) -> .Xr login 1

motd(5) -> .Xr motd 5

This is rendering correct.

I couldn't use this macro in the example title; it doesn't render in the same line.
But the reference is in the next paragraph.

It should read "It is also possible ...", its is a possessive pronoun. :-)

I couldn't use this macro in the example title; it doesn't render in the same line.

Oh, so .Xr references cannot be used with .Ss? Interesting...

it seems so, I couldn't find any other man page doing that.

s/by is/by its/

Perhaps this could be rephrased into

The following command runs
.Ql uname -a
in a jail called
.Dq name .
Since a command is specified explicitly,
.Nm
does not spawn an interactive shell.
Instead,
.Nm
executes the specified command directly.

In that case I'd replace "login(1)" with "the login command" to avoid confusion.

A newline would be nice here.

Usually, .Xr should be on its own line. Perhaps it renders correctly, but it is not how Xr is usually used :)