doc: List identifiers used from header in #if(def) #26972
Description
For "normal" symbols in a file we try to avoid listing them next to the header that makes them available in the file. However, for identifiers used in a #if or #ifdef directive, it could make sense to list them. Especially for config/bitcoin-config.h.
Example:
bitcoin/src/qt/qrimagewidget.cpp
Line 19 in d4c180e
From #26924 (comment)