Sentry buster changes

Thresholds from sig: https://steamcommunity.com/sharedfiles/filedetails/?id=538967086

1st change block:
* Change wording to direct to new section since 15/3k is not true when multiple sentries are deployed.
* The note about buster's detonating other busters was added since they are technically giants, but would *take* 2499 damage. Also the fact this detonation occurs even when they are ubered/in spawn.

2nd change block:
* The wording for "one buster can spawn for each engineer" is misleading because 1) if the engie can create multiple sentries, multiple busters could spawn for him 2) suggests a maximum of 1 buster/engie (gun) when the midair pathing bug can create multiple busters per gun. Added the clarifying info on when multiple busters could spawn.
** The info about 1 buster spawning per spawn wave per sentry was moved to the new section in block 3.
* The wording on "enters the playing area" can be interpreted as leaving the robot spawn zone; clarify it happens shortly after it stops pathing.

3rd change block:
* Added this new section with the spawn information and thresholds based on sig's post at the top of this paste.
* The tooltip on "wave ends" is to clarify the exact event that resets the counters, since it is possible to self-damage in pre-wave and get an instant buster spawn (which wouldn't happen if the counters reset on wave start).