When you using Yelp reading a documentation in DocBook format you can only search the shown page for a word (with Ctrl+F) or searching in all system documentations of a word (search field at the toolbar). But you can't search in the opened document only.

Example: 
1) go to http://typo3.org/documentation/document-library/references/doc_core_tsref/current/  and download the TSref as DocBook
2) extract the zip
3) open the manual.xml in Yelp
4) Try to find the word "wrap"

Result:
1) Pressing Strg+F will "stdWrap" in the shown page the TOC but if you could serach the complete docbook only you will find "wrap" over 30 times
2) entering "wrap" in the search field of the toolbar will not serach the opened document, it will search all system documentations and shwoing you a list of other unrelated files

So there is no way at the moment finding content inside a a Docbook document only.

A solution could be a checkbox besides the searchfield in the toolbar "Only in this document". Another way could be commandline parameter like --thisdoc / -t so Yelp knows that all actions belong to the opened document only.