zsh: document unsupported system-level ZDOTDIR

We rely on temporarily setting ZDOTDIR to our `zsh` resource directory
to implement automatic shell integration. Setting ZDOTDIR in a system
file like /etc/zshenv overrides our ZDOTDIR value, preventing our shell
integration from being loaded.

The only way to prevent /etc/zshenv from being run is via the --no-rcs
flag. (The --no-globalrcs only applies to system-level files _after_
/etc/zshenv is loaded.) Unfortunately, there doesn't appear to be a way
to run a "bootstrap" script (to reimplement the Zsh startup sequence
manually, similar to how our bash integration works) and then enter an
interactive shell session.

https://zsh.sourceforge.io/Doc/Release/Files.html

Given all of the above, document this as an unsupported configuration
for automatic shell integration and point affected users at our manual
shell integration option.

src/shell-integration/README.md

@@ -78,10 +78,16 @@ on the Fish startup process, see the
 
 ### Zsh
 
-For `zsh`, Ghostty sets `ZDOTDIR` so that it loads our configuration
-from the `zsh` directory. The existing `ZDOTDIR` is retained so that
-after loading the Ghostty shell integration the normal Zsh loading
-sequence occurs.
+Automatic [Zsh](https://www.zsh.org/) integration works by temporarily setting
+`ZDOTDIR` to our `zsh` directory. An existing `ZDOTDIR` environment variable
+value will be retained and restored after our shell integration scripts are
+run.
+
+However, if `ZDOTDIR` is set in a system-wide file like `/etc/zshenv`, it will
+override Ghostty's `ZDOTDIR` value, preventing the shell integration from being
+loaded. In this case, the shell integration needs to be loaded manually.
+
+To load the Zsh shell integration manually:
 
 ```zsh
 if [[ -n $GHOSTTY_RESOURCES_DIR ]]; then