Set is_adjusted_to_utc if any timezone set (#1932)

[tustvold]

Which issue does this PR close?

Closes #1932

Rationale for this change

As pointed out by @jorgecarleitao #1937 (comment) we can always set is_adjusted_to_utc if any timezone is set

What changes are included in this PR?

Copies the docstring from arrow schema.fbs into DataType to clarify the potentially unexpected semantics of Timestamp.

Sets is_adjusted_to_utc if any timezone is set

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #1953 (21ef83e) into master (464e8d1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1953   +/-   ##
=======================================
  Coverage   83.47%   83.47%           
=======================================
  Files         221      221           
  Lines       57049    57048    -1     
=======================================
+ Hits        47621    47622    +1     
+ Misses       9428     9426    -2     

Impacted Files	Coverage Δ
arrow/src/datatypes/datatype.rs	65.79% <ø> (ø)
parquet/src/arrow/schema.rs	96.87% <100.00%> (-0.01%)	⬇️
parquet/src/arrow/schema/primitive.rs	76.99% <100.00%> (ø)
parquet/src/encodings/encoding.rs	93.62% <0.00%> (+0.19%)	⬆️
parquet_derive/src/parquet_field.rs	65.98% <0.00%> (+0.22%)	⬆️

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[nevi-me]

The explanation note makes sense