Add an example for using ${RLN}

[ultima]

Summary

Add an example for using ${RLN} for relative paths with description on its use

Reviewed by\: lifanov (mentor), matthew (mentor), mat
Approved by\: lifanov (mentor), matthew (mentor), mat
Differential Revision\: https://reviews.freebsd.org/DXXXXX

[bcr]

Two style comments.

en_US.ISO8859-1/books/porters-handbook/special/chapter.xml

87–92

We use two spaces after a sentence stop.

92

You need to align the line below the <para>, so that the first character of "absolute" is below the first "a" of <para> (see the <para>'s below or above it for an example).

[ultima]

Fixed style comments.

[ultima]

Fixed spacing on programlisting to align with para

[mat]

I rewrote the whole section in D11839.

[ultima]

In D11856#245896, @mat wrote:

I rewrote the whole section in D11839.

Looks perfect