In my QGIS-version (2.18.12, Windows) when opening the expression string builder or the field calculator there is no inline help for the functions. I actually never managed to activate this. Is there an option that I am missing?
For Query Builder, I never saw an inline help, but for Field Calculator maybe it is shifted until the end as you can see below:
You need to go to the far write of the Field Calculator window until the cursor changes its shape to parallel cursor, then drag the window to the left as explained in the image below
Here is the final shape:
If the above solution did not solve your problem, then try to reinstall your QGIS again.