Update documentation for receiver instead of -1

Author: owen-mc

go/ql/lib/semmle/go/dataflow/ExternalFlow.qll

@@ -30,7 +30,7 @@
  *    "Argument[n]", or "Argument[n1..n2]":
  *    - "": Selects a write to the selected element in case this is a field.
  *    - "Argument[n]": Selects an argument in a call to the selected element.
- *      The arguments are zero-indexed, and `-1` specifies the qualifier.
+ *      The arguments are zero-indexed, and `receiver` specifies the receiver.
  *    - "Argument[n1..n2]": Similar to "Argument[n]" but selects any argument
  *      in the given range. The range is inclusive at both ends.
  *
@@ -40,12 +40,12 @@
  *    - "": Selects a read of a selected field.
  *    - "Argument[n]": Selects the post-update value of an argument in a call to the
  *      selected element. That is, the value of the argument after the call returns.
- *      The arguments are zero-indexed, and `-1` specifies the qualifier.
+ *      The arguments are zero-indexed, and `receiver` specifies the receiver.
  *    - "Argument[n1..n2]": Similar to "Argument[n]" but select any argument in
  *      the given range. The range is inclusive at both ends.
  *    - "Parameter": Selects the value of a parameter of the selected element.
  *    - "Parameter[n]": Similar to "Parameter" but restricted to a specific
- *      numbered parameter (zero-indexed, and `-1` specifies the value of `this`).
+ *      numbered parameter (zero-indexed, and `receiver` specifies the receiver).
  *    - "Parameter[n1..n2]": Similar to "Parameter[n]" but selects any parameter
  *      in the given range. The range is inclusive at both ends.
  *    - "ReturnValue": Selects the first value being returned by the selected