user_rounding_error not mentioned in documentation

[gast128@…]

Hello all,

the 'user_rounding_error' is mentioned on the 'Error Handling' page but not on the 'Calling User Defined Error Handlers' page of the Boost.Math.SpecialFunctions documentation.

[John Maddock]

(In [67231]) Improve performance of non-central T by simplifying the arithmetic where possible. Change error handlers to pass the target type down to "rounding errors". Change the rounding error handlers to return a well defined value (the maximum representable value in the target type). Update and complete docs on rounding errors. Fix typo in test/test_beta_dist.cpp. Regenerate the docs. Remove illegal comment from test_constants.cpp. Fixes #4977.