Removed redundant versionadded{} for NDIFF_DIFF. Virtually everything

in this section is new in 2.4, and that's all mentioned already in
versionadded{} thingies at the end of the section.  It hurts readability
to have them after every line <wink>.

Doc/lib/libdoctest.tex

@@ -364,7 +364,6 @@ can also be used in doctest directives (see below).
     contains digit \code{1} where actual output contains letter \code{l},
     a line is inserted with a caret marking the mismatching column
     positions.
-  \versionadded{2.4}
 \end{datadesc}
 
 A "doctest directive" is a trailing Python comment on a line of a doctest