Simplify INPUT rule, document missing prompt string.

Author: dmsc

manual.md

@@ -955,6 +955,7 @@ Console Print and Input Statements
 **INPUT _var_ / I.**  
 **INPUT "prompt"; _var_**  
 **INPUT "prompt", _var_**
+**INPUT ; _var_**
 
   Reads from keyboard/screen and stores
   the value in _var_.
@@ -965,7 +966,9 @@ Console Print and Input Statements
   after the prompt, spaces are printed
   to align to a column multiple of 10
   (similar to how a comma works in
-  PRINT).
+  `PRINT`).  In the case you don't want
+  any prompt, you can use a semicolon
+  alone.
 
   If the value can't be read because
   input errors, the error is stored in

src/syntax/basic.syn

@@ -381,7 +381,7 @@ DIM_MORE: comma
 # INPUT
 INPUT_STR:
         "\"" emit TOK_CSTRING E_CONST_STRING emit TOK_PRINT_STR PRINT_SEP_EOL # Prints a given string
-        PRINT_SEP_EOL                                                         # If starts with ',' or ';', don't print anyting
+        ";"                                         # If starts with ';' don't print anyting
         emit { TOK_BYTE_PUT, 63 }                   # Prints a '?' by default
 
 INPUT_VAR_LIST: