Makefile: use order-only prereq for podman-remote

podman-remote has a dependency on $(SRCBINDIR), because on
Mac and Windows that's a special dir that may not exist.
But depending on a directory means depending on its mtime,
which changes every time a file in it is updated, which
means running 'make' twice in a row will rebuild podman-remote
for no good reason.

Solution: GNU Make has the concept of "order-only" prerequisites,
precisely for this situation. Use it. Since it's an obscure
feature, document it.

UPDATE: This exposed some nasty duplication wrt podman-remote rules.
Clean those up, and add comments to some confusing sections.

Fixes: #14756

(Also, drive-by edit to remove a stray misdocumented non-option)

Signed-off-by: Ed Santiago <santiago@redhat.com>

docs/remote-docs.sh

@@ -6,7 +6,9 @@ PLATFORM=$1                         ## linux, windows or darwin
 TARGET=${2}                         ## where to output files
 SOURCES=${@:3}                      ## directories to find markdown files
 
-# Overridden for testing.  Native podman-remote binary expected filepaths
+# This is a *native* binary, one we can run on this host. (This script can be
+# invoked in a cross-compilation environment, so even if PLATFORM=windows
+# we need an actual executable that we can invoke).
 if [[ -z "$PODMAN" ]]; then
     case $(env -i HOME=$HOME PATH=$PATH go env GOOS) in
         windows)