Clarify and correct Units section for VOUnits

Although section 4.4 referenced VOUnits as the unit syntax,
the summary and examples in the text did not actually conform
to VOUnits syntax.
I have corrected the text and examples, and clarified the
text to say that unit attributes SHOULD conform to VOUnits.
The use of SHOULD rather than MUST here reflects the consensus
of the discussion on the apps mailing list at
http://mail.ivoa.net/pipermail/apps/2023-May/001576.html

I have *not* updated the examples in the "Possible VOTable extensions"
Appendix A, which contains all manner of abominations alongside
non-VOUnits compliant unit attributes.

VOTable.tex

@@ -1140,14 +1140,14 @@ \subsection{Units}
 some physical unit,
 which is specified by the {\attr{unit}}
 attribute of the {\elem{FIELD}}.
-The  syntax of the {\em unit} string is defined in
-the VOUnit specification, \citet{2014ivoa.spec.0523D};
-it is basically written as a string without blanks or spaces,
-where the symbols {\bf.} or {\bf*} indicate a multiplication,
-{\bf/} stands for the division, and no special symbol is required
-for a power.
-Examples are \attrval{unit}{m2} for m$^2$,
-\attrval{unit}{cm-2.s-1.keV-1} for cm$^{-2}$s$^{-1}$keV$^{-1}$,
+The syntax of the {\em unit} string SHOULD conform to
+the VOUnits specification, \citet{2014ivoa.spec.0523D};
+this requires a string without blanks or spaces
+where multiplication is indicated by the symbol ``{\tt.}'',
+division by the symbol ``{\tt/}''
+and exponentiation by the symbol ``{\tt**}''.
+Examples are \attrval{unit}{m**2} for m$^2$,
+\attrval{unit}{cm**-2.s**-1.keV**-1} for cm$^{-2}$s$^{-1}$keV$^{-1}$,
 or \attrval{unit}{erg/s} for erg\,s$^{-1}$.
 
 \subsection{Unified Content Descriptors}
@@ -2304,6 +2304,8 @@ \subsection{Differences Between Versions 1.4 and 1.5}
     \item timescales for calendar epochs.
     \item positioning advice for \attr{ID} and corresponding references.
     \item noting that \elem{RESOURCE} elements can contain \elem{MIVOT} blocks.
+    \item \attr{unit} attribute SHOULD conform to VOUnits,
+          and correct examples accordingly.
   \end{itemize}
 \end{itemize}