handbook: expand information about mergemaster

[linimon]

Summary

mergemaster(8) assumes /usr/src unless you explicitly pass the path via the -m parameter. Since I only rebuild src a few times a year, I manange to forget this part every. Single. Time.

Let's remind folks that do not use the default /usr/src location.

[bcr]

Looks good and it's worthwhile to have in the handbook.
@linimon: Can you commit this yourself or should I do it?

[linimon]

I can commit it myself (I have a doc bit) but I figured I would wait for more review.

[carlavilla]

I converted the patch to AsciiDoc, thanks for the improvement :)