bug#35187: 26.1; call-process should accept a buffer as stdin. or its doc should reference call-process-region

[Ernesto Alfonso]

When looking at the call-process doc, I didn't see a way to pass a
buffer to stdin without creating a temporary file. It looks like
the DESTINATION argument already supports either a file-name or a
buffer, but passing a buffer to INFILE does not seem supported:

    (call-process "cat" (get-buffer "*Messages*") nil)
    *** Eval error ***  Wrong type argument: stringp, #<buffer *Messages*>

Can this be supported? Could call-process-region be merged into
the INFILE argument to call-process?

As a short-term fix, it would be useful if the call-process referred to
call-process-region.

In GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.22.11)
 of 2019-04-01 built on debian-x1g4
Windowing system distributor 'The X.Org Foundation', version 11.0.11902000
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