Man pages matching name fcconfigsubstitutewithpat in section 3:

FcConfigSubstituteWithPat.3.en	debian 10 / libfontconfig1-dev
FcConfigSubstituteWithPat.3.en	debian 11 / libfontconfig-doc
FcConfigSubstituteWithPat.3.en	debian 12 / libfontconfig-doc