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Fix "100% secondary effect" fix, add compatibility

The original fix was bugged (example: erroneous `pop hl` if `.failed` was jumped to).
Replaced with a less invasive fix, keeping all side effects from the original function; it's less intuitive, but more performant - and the tricky bit has been commented.
Also the new fix is more friendly to the compatibility patch I also added, by request of ShinyDragonHunter.

--- a/docs/bugs_and_glitches.md

+++ b/docs/bugs_and_glitches.md

@@ -175,22 +175,36 @@

 ```diff

 -	; BUG: 1/256 chance to fail even for a 100% effect chance,

 -	; since carry is not set if BattleRandom == [hl] == 255

+- 	call BattleRandom

 +	ld a, [hl]

-+	cp 100 percent

-+	jr z, .ok

- 	call BattleRandom

++	sub 100 percent

++	; If chance was 100%, RNG won't be called (carry not set)

++	; Thus chance will be subtracted from 0, guaranteeing a carry

++	call c, BattleRandom

  	cp [hl]

--	pop hl

--	ret c

-+	jr c, .ok

+	pop hl

+	ret c

  .failed

  	ld a, 1

  	ld [wEffectFailed], a

  	and a

-+.ok

-+	pop hl

  	ret

+```

+

+If you wish to keep compatibility with standard Pokémon Crystal, you can disable the fix during link battles by also applying the following edit in the same place:

+

+```diff

++	ld a, [wLinkMode]

++	cp LINK_COLOSSEUM

++	scf ; Force RNG to be called

++	jr z, .nofix ; Don't apply fix in link battles, for compatibility

+	ld a, [hl]

+	sub 100 percent

+	; If chance was 100%, RNG won't be called (carry not set)

+	; Thus chance will be subtracted from 0, guaranteeing a carry

++ .nofix

+	call c, BattleRandom

 ```

 ## Belly Drum sharply boosts Attack even with under 50% HP