Chipset Reference: VIC-IV TEXTYPOS (D04E/F): Document that value must be > 0

[bjotos]

Describe where we can find the problematic topic
Using TEXTYPOS = 0 will cause all text to disappear, which is a known behavior,
see https://discord.com/channels/719326990221574164/977076334297743360/1147845360677564466,
but not documented in the Chipset Reference.

Describe the solution you'd like
This limitation should be added to the description of TEXTYPOS register (after the VIC-IV register table):
" TEXTYPOS Character generator vertical position ( must be > 0 )".

Additional context
Behavior can be reproduced in BASIC with this sequence, both on NTSC and PAL.
POKE $D02F,$47:POKE $D02F,$52:REM ENABLE VIC-IV REGISTERS
POKE $D04E,0:REM TEXT DISAPPEARS
POKE $D04E,1:REM TEXT APPEARS AGAIN (SHIFTED BELOW TOP BORDER)