document "augment, override, replace" keywords

[bam80]

XKB has a bunch of merging machinery for this, with 3 different merge keywords augment, override, replace.

Originally posted by @bluetech in #162 (review)

Could we please document these keywords?

Apparently, they could be present in different contexts.
I couldn't find any references to them in a wild.
Thanks.

[wismill]

At the beginning of the doc you linked there is a link to Ivan Pascal’s documentation. The best doc about the merge modes is this page.

The behavior of the merge modes is not totally clear though; see #209.

[bam80]

That helped, thanks.
But that's on the Web Archive (which even doesn't open here easily), so eventually we would have to take over on the documentation, I think.

[wismill]

@bam80 Again, from the beginning of the doc:

Due to the complexity of the format, this document is still in construction. Some additional resources are: […]

The doc is WIP but this project is seriously undermanned. Feel free to file a MR to improve the doc. Please do not open further issues about topics already marked as TODO in the doc.