Clarify commit summary text for non-issue related commits

[xdje42]

Two non-issue related commits prefix the summary with "iX:". Thus if one is looking at the output of "git log" to find the required format one might come to the wrong conclusion. Make it clear in the docs what the rule is.

Also reformat the text to not be one long line. One could do this in a separate patch, but this is just documentation.