The description of the \pgfuseshading command in subsection 109.3 (Using Shadings) of chapter 109 (Shadings) has an error: the word \pgfbox is mistakenly mentioned instead of \pgftext. The current quote is:
Inserts a previously declared shading into the text. If you wish to use it in a pgfpicture environment, you should put a \pgfbox around it.
It should be:
Inserts a previously declared shading into the text. If you wish to use it in a pgfpicture environment, you should put a \pgftext around it.
p. 1085
Thank you for reporting. This is fixed now.