[basic.fundamental] Clarify that table of minimum integral type widths applies only to standard integral types

Author: randomnetcat

source/basic.tex

@@ -4970,7 +4970,7 @@
 \end{floattable}
 
 \pnum
-The width of each signed integer type
+The width of each standard signed integer type
 shall not be less than the values specified in \tref{basic.fundamental.width}.
 The value representation of a signed or unsigned integer type
 comprises $N$ bits, where N is the respective width.