[SPARK-6336] LBFGS should document what convergenceTol means

[Lewuathe]

LBFGS uses convergence tolerance. This value should be written in document as an argument.

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[srowen]

If linking to Breeze, would be better to link straight to its LBFGS. However I don't know if we want to promise that implementation detail.

Should we also be updating the javadoc for LBFGS.scala and other similar places where this param is documented?

I'm not sure this text helps someone understand what the value is or how to set it -- what are adjusted values? You're right, it's fairly opaque beyond being something relative (rather than something that might need to change with the input). The thing to know is that lower values mean more iterations and the value must be nonnegative. So how about something more like:

Controls how much relative change is still allowed when L-BFGS is considered to converge. Must be nonnegative. Lower values are less tolerant and therefore generally cause more iterations to be run.

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Test build #28648 has started for PR 5033 at commit ac03c3a.

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Test build #28648 timed out for PR 5033 at commit ac03c3a after a configured wait of 120m.

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[srowen]

Nit: works better as "Lower convergence values are less tolerant and therefore ..." as you have elsewhere. (I can get to this on merge if you don't first.) LGTM otherwise. I think we can ignore the Jenkins timeout. I will verify the docs build before merging.

[Lewuathe]

I'm sorry for my unnatural expression. I modified to be more natural. Thank you.

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Test build #28722 has started for PR 5033 at commit e738b33.

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Test build #28722 has finished for PR 5033 at commit e738b33.

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[mengxr]

Merged into master and branch-1.3. Thanks!