minor fix related to Auxiliary processes and IO workers

Hello Andres,


I noticed a comment in "include/miscadmin.h" which might be need a fix:
it states that there is a single auxiliary process of each kind running 
at once.

However, with IO workers it's not true anymore I believe.
See minor patch attached.


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Cédric Villemain +33 6 20 30 22 52
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