Compensate for Q3 frametime-dependent gravity in configs and jump pad code

Adjust jump pad velocity on Q3 maps to match 125fps frametime-dependent behaviour

Corrections and documentation for gravity etc in physicsCPMA.cfg and physicsQ3.cfg

See comments for source of magic number.

Test map: https://ws.q3df.org/map/AMT-Freestyle6/
This has two diagonally opposite jump pads which place the player on the central piller jump pad (or, should). One of these jump pads aims lower than the other, and in Xonotic without this jumppads.qc patch, the player smacks into the side of the pillar (regardless of sv_gravity sv_stepheight and sv_q3compat_changehitbox settings). Note that it's possible to overshoot the centre jump pad from the "higher aiming" jump pad in DeFRaG. !1134 (merged) is also required.

Closes #2187 (closed)