Fix documentation issues for restricted bending potential#

The actual implementation in the code was correct, but the manual section for the restricted bending potential had an extra factor 2 for the force inherited from the 2013 Bulacu JCTC paper, and the journals for two of the references had been swapped. No changes to any simulation results.

Issue 4568

AWH friction metric is shared between AWH walkers#

The friction metric now uses data from all walkers sharing the bias. In the AWH output only the shared friction output is written.

Issue 3842

Required gmx grompp -maxwarn to take a positive integer#

Previously -1 was accepted and suppressed all warnings. Now a positive integer must be chosen.