Minor corrections before I start to update documentation for KMail

Replace locally defined entities with entities from global "general.entities" where possible. Remove extraneous XML tags and regularize references to authors by using items from global "contributor.entities" wherever that is possible. Patch some punctuation.

I'm getting ready to bring the KMail documentation up to date. This patch doesn't change the appearance of the generated .html files, but it does make the .docbook source code a little smaller. I'm doing it this way to get my feet wet with the new "Gitlab" setup. Substantive changes to follow.