(info "(make) Text Functions") says '$(subst FROM,TO,TEXT)' Performs a textual replacement on the text TEXT: each occurrence of FROM is replaced by TO. The result is substituted for the function call. For example,
$(subst ee,EE,feet on the street) produces the value 'fEEt on the strEEt'. OK, but also do mention how to deal with a FROM that has commas in it. Or mention that there is no way to deal with it.