regex.3: Clarify details of matching

The first is that it's far from clear that the end points of the
complete string match are stored in the zero'th element of the
regmatch_t array; secondly, the phrase "next largest substring
match" is positively misleading, implying some sort of size
ordering, whereas in fact they are ordered according to their
appearance in the regex pattern.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/regex.3

@@ -93,7 +93,7 @@ Subsequent
 searches using this pattern buffer will be case insensitive.
 .TP
 .B REG_NOSUB
-Support for substring addressing of matches is not required.
+Do not report position of matches.
 The
 .I nmatch
 and
@@ -163,14 +163,26 @@ above)
 Unless
 .B REG_NOSUB
 was set for the compilation of the pattern buffer, it is possible to
-obtain substring match addressing information.
+obtain match addressing information.
 .I pmatch
 must be dimensioned to have at least
 .I nmatch
 elements.
 These are filled in by
 .BR regexec ()
-with substring match addresses.
+with substring match addresses. The offsets of the subexpression
+starting at the
+.IR i th
+open parenthesis are stored in
+.IR pmatch[i] .
+The entire regular expression's match addresses are stored in
+.IR pmatch[0] .
+(Note that to return the offsets of
+.I N
+subexpression matches,
+.I nmatch
+must be at least
+.IR N+1 .)
 Any unused structure elements will contain the value \-1.
 
 The