subst: Mention how to deal with a FROM that has commas in it

[Dan Jacobson]

(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.