how can I filter for tasks that are all using specific words .. e.g. WF for things other people have to come back to me for
If a task is named "make a phonecall" then you can simply type "phonecall" within the search box in the top-left corner. If the text is too short, though or you want exact matches, please use "q: phonecall".
If you're using labels marking a status of a task, you can also just type in the name of the label, for example - @waiting to the search box.
where is the use of prefixes like "q:" documented?
I was trying to search all tasks with the word "todoist" but the search (or filter) turned up the tasks that are due today as well. The "q:" prefix takes care of it. Are there other prefixes or operators like that ?
You can find this documented here: https://todoist.com/app?v=170#info_page/filters