bug#30419: tramp-sh-process-file: effect of tramp-remote-process-environment vs non-top level value of process-environment

[Noam Postavsky <npostavs@users.sourceforge.net>]

X-Debbugs-CC: Michael Albinus <michael.albinus@gmx.de>
Severity: minor

`(tramp) Remote processes' says:

       TRAMP does not use the defaults specified in `process-environment'
    for running `process-file' or `start-file-process' on remote hosts.
    When values from `process-environment' are needed for remote processes,
    then set them as follows:

         (let ((process-environment (cons "HGPLAIN=1" process-environment)))
           (process-file ...))

       This works only for environment variables not already set in the
    `process-environment'.

I'm confused as to why the environment variables aren't controlled by
tramp-remote-process-environment.  Or is that in addition to the effect
of tramp-remote-process-environment?  Does one take precedence over the
other?