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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docs/reference/settings.md

@@ -2055,11 +2055,11 @@ be correct.
 #### [`exclude-newer`](#pip_exclude-newer) {: #pip_exclude-newer }
 <span id="exclude-newer"></span>
 
-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.
 
 **Default value**: `None`
 
@@ -2071,13 +2071,13 @@ system's configured time zone.
 
     ```toml
     [tool.uv.pip]
-    exclude-newer = "2006-12-02"
+    exclude-newer = "2006-12-02T02:07:43Z"
     ```
 === "uv.toml"
 
     ```toml
     [pip]
-    exclude-newer = "2006-12-02"
+    exclude-newer = "2006-12-02T02:07:43Z"
     ```
 
 ---