Clarify exclude-newer only allows full timestamps in settings docs (#9135)

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## Summary

Follow up to #8553

Clarifies that the `exclude-newer` setting must be a full timestamp and
not a date.

<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan

N/A

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crates/uv-settings/src/settings.rs

@@ -1276,16 +1276,16 @@ pub struct PipOptions {
         "#
     )]
     pub universal: Option<bool>,
-    /// Limit candidate packages to those that were uploaded prior to the given date.
+    /// Limit candidate packages to those that were uploaded prior to a given point in time.
     ///
-    /// Accepts both [RFC 3339](https://www.rfc-editor.org/rfc/rfc3339.html) timestamps (e.g.,
-    /// `2006-12-02T02:07:43Z`) and local dates in the same format (e.g., `2006-12-02`) in your
-    /// system's configured time zone.
+    /// Accepts a superset of [RFC 3339](https://www.rfc-editor.org/rfc/rfc3339.html) (e.g.,
+    /// `2006-12-02T02:07:43Z`). A full timestamp is required to ensure that the resolver will
+    /// behave consistently across timezones.
     #[option(
         default = "None",
         value_type = "str",
         example = r#"
-            exclude-newer = "2006-12-02"
+            exclude-newer = "2006-12-02T02:07:43Z"
         "#
     )]
     pub exclude_newer: Option<ExcludeNewer>,