[lex.ccon] Align char16_t phrasing to UTF-8 one (#1847)

Author: jensmaurer

source/lex.tex

@@ -1156,10 +1156,10 @@
 \indextext{prefix!\idxcode{u}}%
 is a character literal of type \tcode{char16_t}. The value
 of a \tcode{char16_t} character literal containing a single \grammarterm{c-char} is
-equal to its ISO 10646 code point value, provided that the code point is
-representable with a single 16-bit code unit. (That is, provided it is a
-basic multi-lingual plane code point.) If the value is not representable
-within 16 bits, the program is ill-formed. A \tcode{char16_t} character literal
+equal to its ISO 10646 code point value, provided that the code point value is
+representable with a single 16-bit code unit (that is, provided it is in the
+basic multi-lingual plane). If the value is not representable
+with a single 16-bit code unit, the program is ill-formed. A \tcode{char16_t} character literal
 containing multiple \grammarterm{c-char}{s} is ill-formed.
 
 \pnum