[tuple.helper] Paragraph 1 not universally true (cplusplus#6777)

[tuple.helper] p1 defines a general requirement that all specializations of `tuple_size`
shall meet the *Cpp17UnaryTypeTrait* requirements, but p4 actually defines a
special situation where this requirement is not met ("Otherwise, it has no member value").
We have the same seemingly contradiction in [depr.tuple] p2. For clarity, we should
point out that this provision is not universal.

source/utilities.tex

@@ -2730,7 +2730,8 @@
 
 \begin{itemdescr}
 \pnum
-All specializations of \tcode{tuple_size} meet the
+Except where specified otherwise,
+all specializations of \tcode{tuple_size} meet the
 \oldconcept{UnaryTypeTrait} requirements\iref{meta.rqmts} with a
 base characteristic of \tcode{integral_constant<size_t, N>}
 for some \tcode{N}.