Issue #9583: Document startup option/environment interaction.

Patch by Todd Rovito.

Doc/using/cmdline.rst

@@ -379,7 +379,10 @@ Options you shouldn't use
 Environment variables
 ---------------------
 
-These environment variables influence Python's behavior.
+These environment variables influence Python's behavior, they are processed
+before the command-line switches other than -E.  It is customary that
+command-line switches override environmental variables where there is a
+conflict.
 
 .. envvar:: PYTHONHOME
 
@@ -570,4 +573,3 @@ if Python was configured with the ``--with-pydebug`` build option.
 
    If set, Python will print memory allocation statistics every time a new
    object arena is created, and on shutdown.
-

Misc/ACKS

@@ -901,6 +901,7 @@ Hugo van Rossum
 Saskia van Rossum
 Donald Wallace Rouse II
 Liam Routt
+Todd Rovito
 Craig Rowland
 Clinton Roy
 Paul Rubin