I don't think it scales very well to repeat this for every option that might alter the filesystem, because there are a lot:

• everything that mounts a directory will create an empty directory as a mount point if necessary
• everything that mounts a non-directory will create an empty regular file as a mount point if necessary
• --symlink will modify the real filesystem if its destination is a writeable file system
• so will --chmod

This is really a fact about mounting filesystems, not a fact about the command-line options that modify the filesystem. Would it perhaps be clearer what is going on if we say something similar in --bind instead of this? Maybe like this:

        <para>If a process inside the sandbox or a later command-line option
          modifies the contents of <arg choice="plain">DEST</arg>, those
          modifications will affect <arg choice="plain">SRC</arg>
          outside the sandbox.</p>

If --bind said this, would you have assumed that the same was true for --bind-try, --dev-bind and --dev-bind-try without needing it to be said explicitly?

(We certainly don't need to add this to --ro-bind and --ro-bind-try because those are read-only mounts.)

I think it's valuable to use essentially the same language to talk about what happens if a process inside the sandbox does a filesystem write (whether that's symlink() or chmod() or a file overwrite or whatever), and what happens if a subsequent command-line option tells bwrap to do a filesystem write, because they're fundamentally the same operation.

I wouldn't want anyone to get the mistaken idea that a --symlink or --chmod or --file is permanent, but the COMMAND doing an equivalent symlink() or chmod() or open() somehow wouldn't be permanent.

I think it's worth calling out --file specifically, because it's the only one that will silently overwrite an existing file, but I'd prefer it more like perhaps this:

        <para>If <arg choice="plain">DEST</arg> already exists, it will be overwritten.
          Unlike <option>--bind-data</option>, if <arg choice="plain">DEST</arg> or
          a parent directory is a bind-mount from outside the sandbox,
          this may overwrite an existing file outside the sandbox.</para>

Would that have given you the right expectations?

Yes, that sounds great. Shall I apply it to this PR?

Yes please!