docs: Comment about spaces in nix-shell config (#2106)

* Add comment about doublespace in nix-shell config

* docs: Add a semantic PR commit to appease the bot

src/configs/nix_shell.rs

@@ -12,6 +12,9 @@ pub struct NixShellConfig<'a> {
     pub disabled: bool,
 }
 
+/* The trailing double spaces in `symbol` are needed to work around issues with
+multiwidth emoji support in some shells. Please do not file a PR to change this
+unless you can show that your changes do not affect this workaround.  */
 impl<'a> RootModuleConfig<'a> for NixShellConfig<'a> {
     fn new() -> Self {
         NixShellConfig {