z80 asm: document the 0x8000 limitation better

When I get bitten by it myself, it's because it's clearly
underdocumented!

blk.fs

@@ -291,10 +291,11 @@ CREATE lblnext 0 , ( stable ABI until set in B300 )
 ( ----- 026 )
 ( Place BEGIN, where you want to jump back and AGAIN after
   a relative jump operator. Just like BSET and BWR. )
-: BEGIN, PC ;
-: BSET PC SWAP ! ;
+: BEGIN,
+    PC DUP 0x8000 AND IF ABORT" PC must be < 0x8000" THEN ;
+: BSET BEGIN, SWAP ! ;
 ( same as BSET, but we need to write a placeholder )
-: FJR, PC 0 C, ;
+: FJR, BEGIN, 0 C, ;
 : IFZ, JRNZ, FJR, ;
 : IFNZ, JRZ, FJR, ;
 : IFC, JRNC, FJR, ;

doc/asm.txt

@@ -60,6 +60,9 @@ labels, write happen before set. The write operation writes a
 dummy placeholder, and then the set operation writes the offset
 at that placeholder's address.
 
+Important limitation: Flow words are broken when PC reaches
+0x8000. The BREAK, word relies on that 15th bit as a flag.
+
 Variable actions are expected to be called with labels in
 front of them. Examples: