Document that VerbatimUrl does not preserve original string after serialization (#14456)

This came up in
[discussion](https://github.com/astral-sh/uv/pull/14387#issuecomment-3032223670)
on #14387.

crates/uv-pep508/src/verbatim_url.rs

@@ -18,11 +18,16 @@ use uv_redacted::DisplaySafeUrl;
 use crate::Pep508Url;
 
 /// A wrapper around [`Url`] that preserves the original string.
+///
+/// The original string is not preserved after serialization/deserialization.
 #[derive(Debug, Clone, Eq)]
 pub struct VerbatimUrl {
     /// The parsed URL.
     url: DisplaySafeUrl,
     /// The URL as it was provided by the user.
+    ///
+    /// Even if originally set, this will be [`None`] after
+    /// serialization/deserialization.
     given: Option<ArcStr>,
 }