[Minor] Clarify PageIterator Documentation

[Ted-Jiang]

Which issue does this PR close?

Closes #.

Rationale for this change

If i'm right, PageIterator should return one column in multi row group.
I was mislead by this 😂 when i first time read this.

What changes are included in this PR?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #1831 (e8d6db5) into master (db41b33) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1831   +/-   ##
=======================================
  Coverage   83.53%   83.54%           
=======================================
  Files         200      200           
  Lines       56798    56798           
=======================================
+ Hits        47449    47451    +2     
+ Misses       9349     9347    -2     

Impacted Files	Coverage Δ
parquet/src/column/page.rs	98.68% <ø> (ø)
parquet/src/encodings/encoding.rs	93.46% <0.00%> (-0.20%)	⬇️
parquet_derive/src/parquet_field.rs	66.21% <0.00%> (+0.68%)	⬆️

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[tustvold]

I agree the many levels of readers and iterators are deeply confusing 😅 I do hope to clean it up at some point, but the record level APIs make this non-trivial