Re: symbol not coding-system-p
`coding-system-p' does not "call for a coding system,"
Then "calling for a coding system" is behavior of some other
functions, and describing what this behavior involves is irrelevant in
`coding-system-p' docstring and can make people (like me) thing it to
do what it does not.
What do you want the docstring to say? Something like "Use
`find-coding-system' to determine whether a symbol names a coding
system or not"?
Perhaps removing irrelevant text as stated above is enough.
Is it still unreasonable to change `coding-system-p' behavior? It
certainly can make porting elisp easier.