Add exception.escaped, clean up exception attributes javadoc.

[Oberon00]

This adds a semantic attribute key for exception.escaped as introduced in open-telemetry/opentelemetry-specification#784 (updated in open-telemetry/opentelemetry-specification#946).

It also cleans up the javadoc on the existing exception semantic attributes.

[codecov]

Codecov Report

Merging #1672 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1672      +/-   ##
============================================
+ Coverage     86.42%   86.43%   +0.01%     
  Complexity     1384     1384              
============================================
  Files           162      162              
  Lines          5370     5376       +6     
  Branches        554      554              
============================================
+ Hits           4641     4647       +6     
  Misses          522      522              
  Partials        207      207              

Impacted Files	Coverage Δ	Complexity Δ
api/src/main/java/io/opentelemetry/trace/Span.java	100.00% <ø> (ø)	0.00 <0.00> (ø)
.../main/java/io/opentelemetry/common/Attributes.java	96.66% <100.00%> (+0.11%)	17.00 <0.00> (ø)
...o/opentelemetry/common/ImmutableKeyValuePairs.java	98.24% <100.00%> (+0.03%)	24.00 <0.00> (ø)
...telemetry/trace/attributes/SemanticAttributes.java	100.00% <100.00%> (ø)	1.00 <0.00> (ø)
.../java/io/opentelemetry/sdk/resources/Resource.java	90.00% <100.00%> (ø)	14.00 <0.00> (ø)
...ntelemetry/sdk/trace/RecordEventsReadableSpan.java	81.81% <100.00%> (+0.15%)	77.00 <0.00> (ø)

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