Learn about Sphinx and restructured text.
Get documentation source files:
svn co https://svn.plone.org/svn/collective/collective.developermanual/trunk collective.developermanual
Edit the file which you want to update
Check that Sphinx building does not give warnings:
sphinx-build source build
Commit changes:
svn commit -m "My message what I changed"
If you do not want to get commit access you can just email the changed text to mikko.ohtamaa (at) twinapex (dot) com.