Consider mentioning `include.rellinks` in `args.rellinks` docs

[chrispy-snps]

Description

There was a recent discussion in the DITA discussion group about needing more control than args.rellinks provides:

Limiting scope of rellinks

Possible Solution

It might be good to mention in the args.rellinks description that for more control, the internal include.rellinks option can be used (with a link to that topic).