in diskio.f_readline make sure AY result isn't clobbered

compiler/res/prog8lib/cx16/diskio.p8

@@ -436,7 +436,9 @@ _line_end   dey     ; get rid of the trailing end-of-line char
             sta  (P8ZP_SCRATCH_W1),y
 _end        jsr  cbm.READST
             pha
+            phy
             jsr  cbm.CLRCHN
+            ply
             pla
             rts
         }}

compiler/res/prog8lib/shared_cbm_diskio.p8

@@ -390,9 +390,13 @@ _line_end   dey     ; get rid of the trailing end-of-line char
             lda  #0
             sta  (P8ZP_SCRATCH_W1),y
 _end        jsr  cbm.READST
+            pha
+            tya
             pha
             jsr  cbm.CLRCHN
             pla
+            tay
+            pla
             rts
         }}
     }

docs/source/programming.rst

@@ -405,7 +405,7 @@ Directives
       if that can't be found, select the first occurrence regardless. If no other blocks are found, no merge is done. Blocks in libraries are considered first to merge into.
     - ``no_symbol_prefixing`` (block or module) makes the compiler *not* use symbol-prefixing when translating prog8 code into assembly.
       Only use this if you know what you're doing because it could result in invalid assembly code being generated.
-      This option can be useful when writing library modules that you don't want to be exposing prefixed assembly symbols.
+      This option can be useful when writing library modules that you don't want to be exposing prefixed assembly symbols. Various standard library modules use it for this purpose.
     - ``ignore_unused`` (block or module) suppress warnings about unused variables and subroutines. Instead, these will be silently stripped.
       This option is useful in library modules that contain many more routines beside the ones that you actually use.
     - ``verafxmuls`` (block, cx16 target only) uses Vera FX hardware word multiplication on the CommanderX16 for all word multiplications in this block. Warning: this may interfere with IRQs and other Vera operations, so use this only when you know what you're doing. It's safer to explicitly use ``verafx.muls()``.

docs/source/todo.rst

@@ -1,7 +1,6 @@
 TODO
 ====
 
-diskio:  in 1 routine also save Y not only A? it's when it calls CLRCHN
 various library modules: remove %option no_symbol_prefixing?
 
 - DONE: make word arrays split by default and add new @nosplit tag to make an array use the old linear storage format