Apply suggestions from code review

Co-authored-by: Hilary James Oliver <[email protected]>

Author: oliver-sanders

src/reference/changes.rst

@@ -136,7 +136,7 @@ The graph view now has an option to group tasks by cycle point.
 Completion Expressions
 ^^^^^^^^^^^^^^^^^^^^^^
 
-When a task finishes, its outputs are validated against a "completion
+When a task achieves a final status, its outputs are validated against a "completion
 expression" to ensure that it has produced all of its
 :term:`required outputs <required output>`.
 If a task fails this validation check it is said to have "incomplete outputs"
@@ -152,7 +152,7 @@ expression for finer control. This is particularly useful for anyone using
 :term:`custom outputs <custom output>`.
 
 For example, ``mytask`` must produce one of the outputs ``x`` or ``y`` to pass
-the configured completion expression configured here:
+the completion expression configured here:
 
 .. code-block:: cylc
 

src/user-guide/writing-workflows/scheduling.rst

@@ -1898,7 +1898,7 @@ Custom Outputs
 ^^^^^^^^^^^^^^
 
 Branching is particularly powerful when using
-:term:`custom outputs <custom output>` to define multiple parallel paths in the
+:term:`custom outputs <custom output>` to define alternate parallel paths in the
 graph.
 
 In the following graph there is a task called ``showdown`` which produces one
@@ -1990,7 +1990,7 @@ Completion Expressions
 The :cylc:conf:`completion` configuration above is optional, it adds a basic
 validation check which ensures that at least one of the three custom outputs is
 produced when the task runs. This protects you against the possibility that
-none of the outputs are produced e.g. due to a task design error.
+none of the outputs are produced e.g. due to a task implementation error.
 
 If the task does not produce at least one of these three outputs, then it will
 be marked as having incomplete outputs and will be retained in a similar manner