doc: Improve command to generate list of authors for release notes

- Remove dependency on sed (sed was overkill when you can just add plain
  text to the git log --format command)
- Sort resulting list of authors alphabetically (case-insensitive)
- Provide an example of how to only generate authors between versions

Author: MitchellCash

doc/release-process.md

@@ -50,7 +50,7 @@ and sort them into categories based on labels)
 
 Generate list of authors:
 
-    git log --format='%aN' "$*" | sort -ui | sed -e 's/^/- /'
+    git log --format='- %aN' v(current version, e.g. 0.16.0)..v(new version, e.g. 0.16.1) | sort -fiu
 
 Tag version (or release candidate) in git