Add javadoc to Subsegment.close() method.

[Kurru]

Issue #, if available:
#318

Description of changes:
Add javadoc to Subsegment.close() method.
This method should not be used to end subsegments for segments using multiple threads as this can result in a deadlock.

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

[codecov-commenter]

Codecov Report

Merging #342 (fe0bdec) into master (7e7d732) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #342      +/-   ##
============================================
- Coverage     58.96%   58.95%   -0.02%     
  Complexity     1207     1207              
============================================
  Files           131      131              
  Lines          5064     5065       +1     
  Branches        592      592              
============================================
  Hits           2986     2986              
- Misses         1805     1806       +1     
  Partials        273      273              

Impacted Files	Coverage Δ
...n/java/com/amazonaws/xray/emitters/UDPEmitter.java	30.55% <0.00%> (-0.88%)	⬇️
...n/java/com/amazonaws/xray/entities/Subsegment.java	100.00% <ø> (ø)

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[jj22ee]

Hi @Kurru, could you change your javadoc as the .close() method is not designed for external use and should not be used externally in general, rather than for a specific use case?

[Kurru]

I have updated the javadoc as requested. Let me know if this new wording is insufficient.