docs: Point ParquetFormat setter/getter docs to the ParquetOptions definitions

[2010YOUY01]

Which issue does this PR close?

• Part of Clean-up config setter/getter doc comments #25101.

Rationale for this change

Same motivation as #25101: a configuration option's main documentation should live on the config field definition, since the user guide is generated from there. The ParquetFormat setters/getters in datafusion-datasource-parquet wrap fields of ParquetOptions but re-explained them independently. Some of that text was already stale.

What changes are included in this PR?

• Unify config comments
• Update config reference page

What is the testing strategy for this PR?

NA

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.80%. Comparing base (bf01e28) to head (a1d514b).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #25103   +/-   ##
=======================================
  Coverage   81.80%   81.80%           
=======================================
  Files        1130     1130           
  Lines      417707   417754   +47     
  Branches   417707   417754   +47     
=======================================
+ Hits       341708   341749   +41     
- Misses      55875    55879    +4     
- Partials    20124    20126    +2     

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