docs: Improve deprecation of TimeZone::datetime_from_str

[tmccombs]

Add recommending using NaiveDate::parse_from_str as well

Thanks for contributing to chrono!

If your feature is semver-compatible, please target the 0.4.x branch;
the main branch will be used for 0.5.0 development going forward.

Please consider adding a test to ensure your bug fix/feature will not break in the future.

[codecov]

Codecov Report

Merging #1340 (99a0935) into main (be8f2fd) will increase coverage by 1.73%.
Report is 106 commits behind head on main.
The diff coverage is 95.74%.

❗ Current head 99a0935 differs from pull request most recent head 92066bd. Consider uploading reports for the commit 92066bd to get more accurate results

@@            Coverage Diff             @@
##             main    #1340      +/-   ##
==========================================
+ Coverage   91.25%   92.99%   +1.73%     
==========================================
  Files          38       34       -4     
  Lines       17114    16637     -477     
==========================================
- Hits        15617    15471     -146     
+ Misses       1497     1166     -331     

Files	Coverage Δ
src/datetime/serde.rs	88.49% <100.00%> (+0.92%)	⬆️
src/datetime/tests.rs	96.97% <100.00%> (+0.47%)	⬆️
src/format/formatting.rs	92.52% <ø> (-0.02%)	⬇️
src/format/mod.rs	85.18% <ø> (+0.13%)	⬆️
src/format/parsed.rs	96.83% <100.00%> (+0.02%)	⬆️
src/format/scan.rs	99.41% <100.00%> (+0.16%)	⬆️
src/lib.rs	95.58% <100.00%> (ø)
src/month.rs	74.40% <ø> (-0.60%)	⬇️
src/naive/date.rs	96.13% <100.00%> (-0.07%)	⬇️
src/naive/datetime/tests.rs	98.80% <100.00%> (+0.03%)	⬆️
... and 13 more

... and 3 files with indirect coverage changes

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[djc]

This should probably target the 0.4.x branch?

[tmccombs]

I couldn't figure out how to change the branch target, so I opened a new PR.

[pitdicker]

I couldn't figure out how to change the branch target, so I opened a new PR.

You can change the target branch by clicking Edit besides the PR title (took me months to figure out).