Clarify release instructions about when to merge CHANGELOG update

[alamb]

Which issue does this PR close?

Re #1304

Rationale for this change

It was not 100% clear when releasing arrow when the CHANGELOG update PR should be merged. See #1325 (review)

What changes are included in this PR?

Make it clear the CHANGELOG PR should be merged prior to creating the release candidate

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #1370 (42f3e38) into master (5737449) will decrease coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 42f3e38 differs from pull request most recent head 99d6fb6. Consider uploading reports for the commit 99d6fb6 to get more accurate results

@@            Coverage Diff             @@
##           master    #1370      +/-   ##
==========================================
- Coverage   82.98%   82.98%   -0.01%     
==========================================
  Files         181      181              
  Lines       52932    52932              
==========================================
- Hits        43925    43924       -1     
- Misses       9007     9008       +1     

Impacted Files	Coverage Δ
arrow/src/datatypes/field.rs	53.79% <0.00%> (-0.31%)	⬇️
arrow/src/array/transform/mod.rs	84.39% <0.00%> (-0.14%)	⬇️
parquet_derive/src/parquet_field.rs	66.21% <0.00%> (+0.22%)	⬆️

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[alamb]

fyi @nevi-me and @houqp