Remove comment suggesting IonWriter isn't stable

[dlurton]

Inspired by this question: #10 (comment)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #300 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #300      +/-   ##
============================================
+ Coverage     63.87%   63.89%   +0.02%     
- Complexity     4806     4809       +3     
============================================
  Files           136      136              
  Lines         21052    21052              
  Branches       3813     3813              
============================================
+ Hits          13447    13452       +5     
+ Misses         6267     6264       -3     
+ Partials       1338     1336       -2     

Impacted Files	Coverage Δ	Complexity Δ
src/com/amazon/ion/impl/BlockedBuffer.java	51.70% <0.00%> (+0.60%)	81.00% <0.00%> (+3.00%)

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