[lex.separate] Redistribute second comment

The second comment in [lex.separate] is both too specific, and not specific
enough.  Move the last sentence about use of separately translated TUs to
the end of the first note, where it seems most appropriate.  Move the rest
to just after the definition of translation unit in phase 7 of translation.

Author: AlisdairM

source/lex.tex

@@ -39,17 +39,8 @@
 called a \defnadj{preprocessing}{translation unit}.
 \begin{note}
 A \Cpp{} program need not all be translated at the same time.
-\end{note}
-
-\pnum
-\begin{note}
-Previously translated translation units can be preserved individually or in libraries.
-The separate translation units of a program communicate\iref{basic.link} by (for example)
-calls to functions whose identifiers have external or module linkage,
-manipulation of objects whose identifiers have external or module linkage, or
-manipulation of data files. Translation units can be separately
-translated and then later linked to produce an executable
-program\iref{basic.link}.
+Translation units can be separately translated and then later linked
+to produce an executable program\iref{basic.link}.
 \end{note}
 \indextext{compilation!separate|)}
 
@@ -207,6 +198,13 @@
 entities and any external representation. The description is conceptual
 only, and does not specify any particular implementation.
 \end{note}
+\begin{note}
+Previously translated translation units can be preserved individually or in libraries.
+The separate translation units of a program communicate\iref{basic.link} by (for example)
+calls to functions whose identifiers have external or module linkage,
+manipulation of objects whose identifiers have external or module linkage, or
+manipulation of data files.
+\end{note}
 
 While the tokens constituting translation units
 are being analyzed and translated,