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

Author: lewuathe <lewuathe@me.com>

Closes #5033 from Lewuathe/SPARK-6336 and squashes the following commits:

e738b33 [lewuathe] Modify text to be more natural
ac03c3a [lewuathe] Modify documentations
6ccb304 [lewuathe] [SPARK-6336] LBFGS should document what convergenceTol means