[R6RS] naming convention for procedures

[Anton van Straaten]

I notice that the "Entry format" section (3.1) does not list "proc" or 
"procedure" as a naming convention which implies a type restriction. 
I'm thinking it should.

Currently, both "proc" and "procedure" are used for this purpose in 
different places - "procedure" most notably in arithmetic and hash 
tables.  R5RS used "proc" for e.g. map and for-each (and similarly 
didn't mention it in the "Entry format" section.

I'm going with "proc" for the stuff I'm responsible for, until I hear 
otherwise.