Documentation fix for new exponential target function.
Also added new function to estimate R2eff and i0 parameters, before minimisation.
task #7822(https://gna.org/task/index.php?7822): Implement user function to estimate R2eff and associated errors for exponential curve fitting.
Authored by: tlinnet 2014-08-24
Parent: [r25234]
Child: [r25236]
