Slightly improve serde documentation

[pitdicker]

Fixes #1484.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 91.81%. Comparing base (236b7ad) to head (2d2062f). Report is 1 commits behind head on main.

Files	Patch %	Lines
src/datetime/serde.rs	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1519 +/- ## ======================================= Coverage 91.81% 91.81% ======================================= Files 40 40 Lines 18351 18355 +4 ======================================= + Hits 16849 16853 +4 Misses 1502 1502 ```

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[pitdicker]

No clue yet why a warning shows up for the naive::time module on this PR but not on main:

error: redundant explicit link target
   --> src/naive/time/mod.rs:193:18
    |
193 | /// [`DateTime`](crate::DateTime) with fractional minutes as offset, or using
    |      ----------  ^^^^^^^^^^^^^^^ explicit target is redundant
    |      |
    |      because label contains path that resolves to same destination
    |
    = note: when a link's destination is not specified,
            the label is used to resolve intra-doc links
    = note: `-D rustdoc::redundant-explicit-links` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(rustdoc::redundant_explicit_links)]`
help: remove explicit link target
    |
193 | /// [`DateTime`] with fractional minutes as offset, or using
    |     ~~~~~~~~~~~~

[pitdicker]

No clue yet why a warning shows up for the naive::time module on this PR but not on main:

Seems to be a bug in the error reporting of rustdoc. The redundant target was in libs.rs but for a similar DateTime link.