Adds documentation regarding thread safety of CompilerPipeline

[alancai98]

Adds to the kdoc, detailing how the builder instance of CompilerPipeline is not thread safe.

Possible TODOs:

• Should we also document this for the CompilerPipeline interface (not just the builder instance)?
• Should we add annotations of non-thread safety?
• Should we also document that EvaluatingCompiler is not thread safe?

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

[dlurton]

• Should we also document this for the CompilerPipeline interface (not just the builder instance)?
  • I would, yes.
• Should we add annotations of non-thread safety?
  • Of what annotations specifically do you speak of? I am not aware of any that aren't part of another library and I'd rather avoid taking a dependency on a new library just for that.
• Should we also document that EvaluatingCompiler is not thread safe?
  • EvaluatingCompiler is internal, but you can add a quick note to it if you'd like.

[alancai98]

New revision includes updated kdoc to CompilerPipeline builder and interface. Also adds a note about thread safety to EvaluatingCompiler.

Regarding

Of what annotations specifically do you speak of? I am not aware of any that aren't part of another library and I'd rather avoid taking a dependency on a new library just for that.

None in particular. I wasn't sure if there was any standard annotation to denote a class was not thread safe.