Update flags documentation:

- new ammo effect flags.
- alternative body part names (left / right body part)
- removed techniques, completely jsonized
- diesel

Fixes #9973

> it looks like it's also intended to work this way on monsters (specifically, the Nether monsters that melt away should never gib).

The flag for monsters is called "NO_GIBS", it is documented in JSON_FLAGS.md and it works fine.

However they could probably change to a common string, what should it be:
- `NO_GIBS`
- `NO_GIB`
- `NOGIB`
- `NOGIBS`

Also the list of ammo types should probably be removed from this file. Ammo types are defined in json completely, listing them here is just pointless and likely outdated redundancy.

Mine, documented in monattack.h but not here.  Guilty.