chore:add javadoc about being called within single thread

[rahulKQL]

Fixes #859

[codecov]

Codecov Report

Merging #856 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #856   +/-   ##
=========================================
  Coverage     78.62%   78.62%           
  Complexity     1163     1163           
=========================================
  Files           203      203           
  Lines          5142     5142           
  Branches        413      413           
=========================================
  Hits           4043     4043           
  Misses          925      925           
  Partials        174      174

Impacted Files	Coverage Δ	Complexity Δ
...n/java/com/google/api/gax/grpc/GrpcStatusCode.java	95.45% <0%> (ø)	20% <0%> (ø)	⬇️
...gle/api/gax/httpjson/HttpJsonTransportChannel.java	47.05% <0%> (ø)	5% <0%> (ø)	⬇️
.../com/google/api/gax/grpc/GrpcTransportChannel.java	59.09% <0%> (ø)	9% <0%> (ø)	⬇️
...m/google/api/gax/batching/FlowControlSettings.java	78.57% <0%> (ø)	2% <0%> (ø)	⬇️
...ava/com/google/api/gax/retrying/RetrySettings.java	31.25% <0%> (ø)	2% <0%> (ø)	⬇️
...le/api/gax/grpc/GrpcUnaryRequestParamCallable.java	90% <0%> (ø)	3% <0%> (ø)	⬇️
...m/google/api/gax/httpjson/HttpRequestRunnable.java	54.16% <0%> (ø)	5% <0%> (ø)	⬇️
.../java/com/google/longrunning/OperationsClient.java	73.33% <0%> (ø)	16% <0%> (ø)	⬇️
...om/google/longrunning/stub/GrpcOperationsStub.java	89.65% <0%> (ø)	11% <0%> (ø)	⬇️
...oogle/api/gax/httpjson/HttpJsonDirectCallable.java	88.23% <0%> (ø)	4% <0%> (ø)	⬇️
... and 16 more

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