Document iterator filters are considered as decisions

Iterator filters, introduced with Ada 2022, can control the flow
of the program. As such when adding support for this construct, the
tool now considers their predicate as decisions for decision level
coverage.

This change documents this in the user guide.

Author: leocreuse

doc/gnatcov/cov_source.rst

@@ -777,6 +777,10 @@ performs Statement and Decision Coverage assessments combined together.
 * The iteration predicate of Ada quantified expressions, in the same
   contexts as for if-expressions.
 
+* The iterator filter predicate optionally present in a loop parameter
+  specification or in an iterator specification, in the same context as
+  if-expressions.
+
 The expression may be of essentially any type in C. In Ada, this may
 be the standard Boolean type, or subtypes of it, or derived types
 thereof. Here are a few examples show-casing decisions in a variety of
@@ -793,6 +797,9 @@ contexts::
   Z := (for all X of Container => P(X));  -- "P(X)", quantified-expression predicate
   T := (for some X of Container => P(X));
 
+  for I in 1 .. 10 when (I mod 2) = 0 loop ... -- "(I mod 2) = 0", iterator filter predicate
+  F := (for Elt of Container when Pred (Elt) => Elt); -- "Pred (Elt)", iterator filter predicate
+
 
 The coverage status of a decision obligation is determined as
 follows: