nixos/manual: Clarify execute exit status

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

nixos/doc/manual/development/writing-nixos-tests.section.md

@@ -166,7 +166,7 @@ The following methods are available on machine objects:
         command as a whole will fail.
 
     -   For pipelines, the last non-zero exit status will be returned
-        (if there is one, zero will be returned otherwise).
+        (if there is one; otherwise zero will be returned).
 
     -   Dereferencing unset variables fails the command.
 

nixos/doc/manual/from_md/development/writing-nixos-tests.section.xml

@@ -291,8 +291,8 @@ start_all()
             <listitem>
               <para>
                 For pipelines, the last non-zero exit status will be
-                returned (if there is one, zero will be returned
-                otherwise).
+                returned (if there is one; otherwise zero will be
+                returned).
               </para>
             </listitem>
             <listitem>