general: Update documentation better qualifying runcom paths

Zsh and Prezto files don't always reside in `~/`. Instead, they have
a more spec compliant location (`${ZDOTDIR:-$HOME}/`). Make them
unambiguous in the documentation. Likewise, for `$GNUPGHOME`.

Further, add instruction for optionally setting up Prezto in
`$XDG_CONFIG_HOME`.

While at this, also add clarification on relative ordering of some of
the modules, apply more formatting tweaks and doc improvements.

modules/macports/README.md

@@ -14,7 +14,7 @@ Defines MacPorts aliases and adds MacPorts directories to path variables.
 
 ## Authors
 
-*The authors of this module should be contacted via the [issue tracker][1].*
+_The authors of this module should be contacted via the [issue tracker][1]._
 
 - [Matt Cable](https://github.com/curiousstranger)
 - [Sorin Ionescu](https://github.com/sorin-ionescu)