Adds recommendation to use IonElement

[popematt]

Issue #, if available:

None

Description of changes:

Recommends using IonElement instead of IonValue.

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

[tgregg]

Should we also put this note in the JavaDoc overview for IonValue?

[popematt]

Should we also put this note in the JavaDoc overview for IonValue?

Yes, i can do that.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.28%. Comparing base (3c1b6b1) to head (709cf24).
Report is 21 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #772      +/-   ##
============================================
+ Coverage     67.23%   67.28%   +0.05%     
- Complexity     5484     5491       +7     
============================================
  Files           159      159              
  Lines         23025    23026       +1     
  Branches       4126     4124       -2     
============================================
+ Hits          15481    15494      +13     
+ Misses         6262     6256       -6     
+ Partials       1282     1276       -6     

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