A big capacitor across the motor terminals will eventually destroy the H-bridge and
the capacitor!
Usually a small value of capacitance across the motor is added to reduce RFI from the
brushes arcing, perhaps 10nF to 100nF. The large capacitance should be decoupling
on the DC supply to the bridge, this can and should be lots of capacitance to reduce
radiating EMI from the supply leads, and bolster the supply when under brief overload
(such as a sudden start). Such decoupling should be in the form of multiple high
ripple-current capacitors to minimize heating of the capacitors themselves.