docs: Fix rendering of get_record_batch_memory_size buffer diagram

[alamb]

Which issue does this PR close?

• N/A (minor doc fix). Follow on to fix: frame IPC messages when reading spill files so batches pin only their own bytes #24592, which I noticed while reviewing that PR.

Rationale for this change

The docs for get_record_batch_memory_size render poorly on docs.rs:

1. The ASCII-art buffer diagram is not in a code block, so rustdoc collapses it into a single unreadable line
2. A stray backtick in Current `RecordBatch`.get_array_memory_size()` will ... causes the rest of the paragraph to render as inline code

The diagram currently renders as this single line:

{xxxxxxxxxxxxxxxxxxx} <— buffer ^ ^ ^ ^ | | | | col1->{ } | | col2———>{ }

What changes are included in this PR?

• Wrap the diagram in a text code fence so it renders as drawn
• Replace the mis-quoted method reference with an intra-doc link to RecordBatch::get_array_memory_size

Are these changes tested?

Yes: RUSTDOCFLAGS="-D warnings" cargo doc -p datafusion-common --no-deps passes (this also verifies the new intra-doc link resolves).

Are there any user-facing changes?

Documentation only.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.43%. Comparing base (26b40dd) to head (df47c1e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24637      +/-   ##
==========================================
- Coverage   81.43%   81.43%   -0.01%     
==========================================
  Files        1118     1118              
  Lines      399414   399414              
  Branches   399414   399414              
==========================================
- Hits       325278   325260      -18     
- Misses      55145    55155      +10     
- Partials    18991    18999       +8     

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[nuno-faria]

Looks good now:

[nuno-faria]

I wonder if this is still needed, since the issue has been completed.

[alamb]

That is an excellent point -- I will remove it in a follow on PR

[alamb]

• Follow on PR docs: Remove stale "temporary fix" note from get_record_batch_memory_size #24674